AzdoBranchCleanupReport.cs

using Microsoft.TeamFoundation.SourceControl.WebApi;
using Microsoft.TeamFoundation.WorkItemTracking.WebApi;
using Microsoft.TeamFoundation.WorkItemTracking.WebApi.Models;
using Microsoft.VisualStudio.Services.Common;
using Microsoft.VisualStudio.Services.WebApi;
using System.Reactive.Linq;
using System.Text.RegularExpressions;

public sealed partial class Program
{
    // Expected args: a single URL that includes /_git
    // This will be the repo whose branches get reported on.
    public static async Task Main(string[] args)
    {
        var (collectionUrl, projectName, repoName) = ParseRepoUrl(args.Single());

        using var connection = new VssConnection(new Uri(collectionUrl), new VssCredentials());

        using var gitClient = connection.GetClient<GitHttpClient>();

        if (!Guid.TryParse(repoName, out var repositoryId))
            repositoryId = (await gitClient.GetRepositoryAsync(projectName, repoName)).Id;

        var branches = await gitClient.GetBranchesAsync(repositoryId);

        using var wiClient = connection.GetClient<WorkItemTrackingHttpClient>();

        var wiqlResult = await wiClient.QueryByWiqlAsync(new Wiql { Query = "SELECT [System.Id] FROM WorkItems WHERE [System.ExternalLinkCount] > 0" });

        var workItems = await wiqlResult.WorkItems
            .Select(wi => wi.Id)
            .Batch(200)
            .ToObservable()
            .SelectMany(async workItemIdBatch => await wiClient.GetWorkItemsAsync(workItemIdBatch, expand: WorkItemExpand.Relations))
            .SelectMany(workItemBatch => workItemBatch)
            .ToList();

        var remainingBranches = branches.ToDictionary(branch => branch.Name);

        Console.WriteLine("## Branches with work items");
        Console.WriteLine();

        foreach (var workItemsByBranchName in
            from workItem in workItems
            from relation in workItem.Relations
            where relation.Rel == "ArtifactLink" && relation.Attributes["name"] is "Branch"
            let branch = ParseBranchUrl(relation.Url)
            where branch.RepositoryId == repositoryId
            group workItem by branch.BranchName into workItemsByBranchName
            orderby workItemsByBranchName.Key
            select workItemsByBranchName)
        {
            if (!remainingBranches.Remove(workItemsByBranchName.Key, out var branch))
                continue;

            Console.WriteLine($"{workItemsByBranchName.Key}:");
            Console.WriteLine($"  -{branch.BehindCount} +{branch.AheadCount}, last updated {branch.Commit.Committer.Date.ToLocalTime()} by {branch.Commit.Committer.Name}");

            foreach (var workItem in workItemsByBranchName)
                Console.WriteLine($"  {collectionUrl}/{projectName}/_workitems/edit/{workItem.Id!.Value} ({workItem.Fields["System.State"]})");

            Console.WriteLine();
        }

        Console.WriteLine("## Branches without work items");
        Console.WriteLine();

        foreach (var branch in remainingBranches.Values.OrderBy(branch => branch.Name))
        {
            Console.WriteLine($"{branch.Name}:");
            Console.WriteLine($"  -{branch.BehindCount} +{branch.AheadCount}, last updated {branch.Commit.Committer.Date.ToLocalTime()} by {branch.Commit.Committer.Name}");
            Console.WriteLine();
        }
    }

    [GeneratedRegex("/(?<project>[^/]+)/_git(/(?<repo>[^/]+))?", RegexOptions.IgnoreCase)]
    private static partial Regex GetRepoUrlRegex();

    private static (string CollectionUrl, string ProjectName, string RepoName) ParseRepoUrl(string url)
    {
        var match = GetRepoUrlRegex().Match(url);
        if (!match.Success)
            throw new ArgumentException("Invalid repository URL format.", nameof(url));

        var projectGroup = match.Groups["project"];
        var repoGroup = match.Groups["repo"];
        return (
            url[..(projectGroup.Index - 1)],
            projectGroup.Value,
            repoGroup.Success ? repoGroup.Value : projectGroup.Value);
    }

    private static (Guid RepositoryId, string BranchName) ParseBranchUrl(string url)
    {
        var splitBeforeRepoId = url.IndexOf("%2F", StringComparison.OrdinalIgnoreCase);
        if (splitBeforeRepoId == -1)
            throw new ArgumentException("Invalid branch URL format.", nameof(url));

        var repoIdStart = splitBeforeRepoId + "%2F".Length;

        var splitAfterRepoId = url.IndexOf("%2F", repoIdStart, StringComparison.OrdinalIgnoreCase);
        if (splitAfterRepoId == -1)
            throw new ArgumentException("Invalid branch URL format.", nameof(url));

        var gitNameStart = splitAfterRepoId + "%2F".Length;
        if (!url.AsSpan(gitNameStart).StartsWith("GB", StringComparison.OrdinalIgnoreCase))
            throw new ArgumentException("Invalid branch URL format.", nameof(url));

        return (
            Guid.Parse(url.AsSpan()[repoIdStart..splitAfterRepoId]),
            url[(gitNameStart + "GB".Length)..].Replace("%2F", "/", StringComparison.OrdinalIgnoreCase));
    }
}

AzdoBranchCleanupReport.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net9.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.TeamFoundationServer.Client" Version="19.225.1" />
    <PackageReference Include="System.Reactive" Version="6.0.1" />
  </ItemGroup>

</Project>