Really Simple Robots.txt editor for WordPress (doesn't require DISALLOW_FILE_EDIT or DISALLOW_FILE_MODS to be false)

robots-txt-additions.php

<?php
/**
 * Plugin Name: Robots.txt Additions
 * Description: Add custom rules to robots.txt
 * Version: 1.0
 * Author: Joseph Roberts
 * Author URI: https://github.com/broskees
 * License: MIT
 * License URI: https://opensource.org/licenses/MIT
 **/


if (! defined('ABSPATH')) {
	exit;
}

new class
{
	private const ROBOTS_TXT_OPENER = '### WP ROBOTS.TXT ADDITIONS ###';
	private const ROBOTS_TXT_CLOSER = '### END WP ROBOTS.TXT ADDITIONS ###';
	private const OPTION_NAME = 'robots_txt_additions';

	public function __construct()
	{
		add_filter('robots_txt', [$this, 'robotsTxtFilter'], PHP_INT_MAX);
		add_action('admin_init', [$this, 'registerSettings']);
	}

	public function robotsTxtFilter(string $output): string
	{
		$additions = get_option(self::OPTION_NAME);
		
		if (! $additions) {
			return $output;
		}

		$openerPos = strpos($output, self::ROBOTS_TXT_OPENER);
		$closerPos = strpos($output, self::ROBOTS_TXT_CLOSER);

		if ($openerPos !== false && $closerPos !== false) {
			$output = substr_replace(
				$output,
				$additions,
				$openerPos + strlen(self::ROBOTS_TXT_OPENER),
				$closerPos - $openerPos - strlen(self::ROBOTS_TXT_OPENER)
			);
			return $output;
		}

		$output .= "\n" . self::ROBOTS_TXT_OPENER . "\n";
		$output .= $additions . "\n";
		$output .= self::ROBOTS_TXT_CLOSER . "\n";

		return $output;
	}

	public function registerSettings()
	{
		add_settings_section(
			'robots_txt_additions_section',
			'Robots.txt Additions',
			'__return_false',
			'reading'
		);

		add_settings_field(
			self::OPTION_NAME,
			'Custom Rules',
			[$this, 'settingsField'],
			'reading',
			'robots_txt_additions_section'
		);

		register_setting('reading', self::OPTION_NAME);
	}

	public function settingsField()
	{
		$additions = get_option(self::OPTION_NAME);
		printf(
			'<textarea id="%1$s" name="%1$s" rows="10" cols="50">%2$s</textarea><p class="description">%s</p>',
			self::OPTION_NAME,
			esc_textarea($additions),
			'Enter your custom robots.txt rules here.'
		);
	}
};

Installing with composer:

{
  "repositories": [
    {
      "type": "package",
      "package": {
        "name": "broskees/robots-txt-additions",
        "version": "dev-main",
        "source": {
          "url": "[email protected]:5fab22818cf6da06dbaa418f891839a1.git",
          "type": "git",
          "reference": "main"
        },
        "type": "wordpress-plugin"
      }
    }
  ],
  "require": {
    "broskees/robots-txt-additions": "dev-main",
  }
}