Pendant for the https://github.com/marijnz/unity-toolbar-extender just for the bottom status bar. Add

UnityAppStatusBarExtender.cs

/*
 * This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>
 */

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;

namespace UnityAppStatusBarExtender
{
	public class AppStatusBarCallback
	{
		public static class Styles
		{
			public static readonly GUIStyle background = "AppToolbar";
			public static readonly GUIStyle statusIcon = "StatusBarIcon";
		}


		static Type m_statusbarType     = typeof(Editor).Assembly.GetType("UnityEditor.AppStatusBar");
		static Type m_consoleWindowType = typeof(Editor).Assembly.GetType("UnityEditor.ConsoleWindow");
		static Type m_ViewType          = typeof(Editor).Assembly.GetType("UnityEditor.View");
#if UNITY_2023_1_OR_NEWER
		static Type m_ShortcutHelperWindowType =
			typeof(ShortcutManager).Assembly.GetType("UnityEditor.ShortcutManagement.HelperWindow");
#endif

		// sadly the AppStatusBar still uses IMGUI... so there is no easy way to inject stuff after some VisualElement
		// that's why we need to re-create the "OldOnGUI()" method https://github.com/Unity-Technologies/UnityCsReference/blob/master/Editor/Mono/GUI/AppStatusBar.cs
		// while adding our InjectionPoints.
		// We can do so using Detours: (https://tryfinally.dev/detours-redirecting-csharp-methods-at-runtime)
    // needs to be put in an asmdef that allows unsafe code, though.

		// so ... get all the used members:
		private static MethodInfo   m_OldOnGUIMethodInfo                  = null;
		private static MethodInfo   m_ConsoleWindowLoadIconsMethodInfo    = null;
		private static MethodInfo   m_ProgressGetMaxElapsedTimeMethodInfo = null;
		private static FieldInfo    m_ShowProgresFieldInfo                = null;
		private static PropertyInfo m_PositionPropertyInfo                = null;
		private static MethodInfo   m_DrawStatusTextMethodInfo            = null;
		private static MethodInfo   m_StatusBarShortcutsMethodInfo        = null;
		private static MethodInfo   m_DrawSpecialModeLabelMethodInfo      = null;
		private static MethodInfo   m_DrawProgressBarMethodInfo           = null;
		private static MethodInfo   m_DrawDebuggerToggleMethodInfo        = null;
		private static FieldInfo    m_DrawExtraFeaturesFieldInfo          = null;
		private static MethodInfo   m_DrawCacheServerToggleMethodInfo     = null;
		private static MethodInfo   m_DrawBakeModeMethodInfo              = null;
		private static MethodInfo   m_DrawRefreshStatusMethodInfo         = null;
		private static MethodInfo   m_ShowRepaintsMethodInfo                          = null;

		const float k_ShowProgressThreshold = 0.5f;


		public static Action OnBeforeProgress;
		public static Action OnAfterProgress;
		public static Action OnAfterButtons;


		static AppStatusBarCallback()
		{
			// TODO: all static variables
			var bfIPP = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
			var bfSPP = BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic;

			m_OldOnGUIMethodInfo = m_statusbarType.GetMethod("OldOnGUI", bfIPP);
			m_ConsoleWindowLoadIconsMethodInfo = m_consoleWindowType.GetMethod("LoadIcons", bfSPP);
			m_ProgressGetMaxElapsedTimeMethodInfo = typeof(Progress).GetMethod("GetMaxElapsedTimeMethodInfo", bfSPP);
			m_ShowProgresFieldInfo = m_statusbarType.GetField("m_ShowProgress", bfIPP);
			m_PositionPropertyInfo = m_ViewType.GetProperty("position", bfIPP);
			m_DrawStatusTextMethodInfo = m_statusbarType.GetMethod("DrawStatusText", bfIPP);
#if UNITY_2023_1_OR_NEWER
			m_StatusBarShortcutsMethodInfo = m_ShortcutHelperWindowType.GetMethod("StatusBarShortcuts", bfSPP);
#endif
			m_DrawSpecialModeLabelMethodInfo = m_statusbarType.GetMethod("DrawSpecialModeLabel", bfIPP);
			m_DrawProgressBarMethodInfo = m_statusbarType.GetMethod("DrawProgressBar", bfIPP);
			m_DrawDebuggerToggleMethodInfo = m_statusbarType.GetMethod("DrawDebuggerToggle", bfIPP);
			m_DrawExtraFeaturesFieldInfo = m_statusbarType.GetField("m_DrawExtraFeatures", bfIPP);
			m_DrawCacheServerToggleMethodInfo = m_statusbarType.GetMethod("DrawCacheServerToggle", bfIPP);
			m_DrawBakeModeMethodInfo = m_statusbarType.GetMethod("DrawBakeMode", bfIPP);
			m_DrawRefreshStatusMethodInfo = m_statusbarType.GetMethod("DrawRefreshStatus", bfIPP);
			m_ShowRepaintsMethodInfo = typeof(EditorGUI).GetMethod("ShowRepaints", bfSPP);

			//EditorApplication.update -= OnUpdate;
			//EditorApplication.update += OnUpdate;

			var replacementMethod =
				DetourUtility.MethodInfoForMethodCall(() => OldOnGUIReplacement(null));

			// DetourUtility.TryDetourFromTo(m_OldOnGUIMethodInfo, replacementMethod);
		}


		// when the editor recompiles, this method throws quite a few NullReferenceExceptions
		static void OldOnGUIReplacement(object appStatusBarSelf)
		{
			m_ConsoleWindowLoadIconsMethodInfo.Invoke(null, Array.Empty<object>());

			// GUI.color = EditorApplication.isPlayingOrWillChangePlaymode ? HostView.kPlayModeDarken : Color.white;

			var position = (Rect) m_PositionPropertyInfo.GetValue(appStatusBarSelf, Array.Empty<object>());

			if (Event.current.type == EventType.Layout)
			{
				var showProgress = Progress.running
				                   && ((float) m_ProgressGetMaxElapsedTimeMethodInfo.Invoke(null,
					                   Array.Empty<object>())) > k_ShowProgressThreshold;

				m_ShowProgresFieldInfo.SetValue(appStatusBarSelf, showProgress);
			}
			else if (Event.current.type == EventType.Repaint)
			{
				Styles.background.Draw(new Rect(0, 0, position.width, position.height), false, false, false, false);
			}


			GUILayout.BeginHorizontal(GUILayout.MaxWidth(position.width));
			{
				GUILayout.Space(2);
				m_DrawStatusTextMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());

				#if UNITY_2023_1_OR_NEWER
				if (EditorPrefs.GetBool("EnableHelperBar", false))
				{
					m_StatusBarShortcutsMethodInfo.Invoke(null, Array.Empty<object>());
				}
				else
				#endif
				GUILayout.FlexibleSpace();


				SafeInvoke(OnBeforeProgress);

				var drawExtraFeatures = (bool) m_DrawExtraFeaturesFieldInfo.GetValue(appStatusBarSelf);
				if (drawExtraFeatures)
					m_DrawSpecialModeLabelMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());

				m_DrawProgressBarMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());

				SafeInvoke(OnAfterProgress);

				m_DrawDebuggerToggleMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());

				if (drawExtraFeatures)
				{
					m_DrawCacheServerToggleMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());
					m_DrawBakeModeMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());
				}

				m_DrawRefreshStatusMethodInfo.Invoke(appStatusBarSelf, Array.Empty<object>());
				SafeInvoke(OnAfterButtons);
			}
			GUILayout.EndHorizontal();

			m_ShowRepaintsMethodInfo.Invoke(null, Array.Empty<object>());
		}

		private static void SafeInvoke(Action arg)
		{
			try
			{
				arg?.Invoke();
			}
			catch(Exception e)
			{
				if(GUILayout.Button(EditorGUIUtility.IconContent("console.warnicon"), AppStatusBarCallback.Styles.statusIcon))
				{
					Debug.LogError(e);
				}
			}
		}
	}
}

|MyStatusBarExtensions.cs

// usage example: 

using System;
using System.Reflection;
using UnityAppStatusBarExtender;
using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public static class MyStatusBarExtensions
{
	
	static MyStatusBarExtensions()
	{
		AppStatusBarCallback.OnAfterButtons += OnAfterProgress;
	}

	private static void OnAfterProgress()
	{
		var content = EditorGUIUtility.TrIconContent("d_Exposure");

		using (new EditorGUIUtility.IconSizeScope(new Vector2(16, 16)))
		{
			if(GUILayout.Button(content, AppStatusBarCallback.Styles.statusIcon))
			{
				Debug.Log("Hey");
			}

		}

	}
}