Using Nuget.Protocol to delete a package from Azure DevOps

Question

I'm writing a simple application to implement package retention in our AzureDevOps NuGet feed, but I can't delete a package -- nothing happens.

(I want to keep all release packages, but delete all pre-release packages for branches that nolonger exist and keep only the latest three pre-release packages for branches that do exist.)

            // Log in and whatnot.
            PackageSource packageSource = new PackageSource("https://pkgs.dev.azure.com/Organisation/_packaging/Organisation/nuget/v3/index.json")
            {
                Credentials = new PackageSourceCredential(
                    source: "https://pkgs.dev.azure.com/Organisation/_packaging/Organisation/nuget/v3/index.json",
                    username: "[email protected]",
                    passwordText: _Pat,
                    isPasswordClearText: true,
                    validAuthenticationTypesText: null)
            };
            NuGet.Protocol.Core.Types.SourceRepository repository = NuGet.Protocol.Core.Types.Repository.Factory.GetCoreV3(packageSource);
            PackageSearchResource packageSearch = repository.GetResourceAsync<NuGet.Protocol.Core.Types.PackageSearchResource>().Result;
            PackageUpdateResource packageUpdate =  repository.GetResourceAsync<PackageUpdateResource>().Result;

// Delete a package -- doesn't work!
packageUpdate.Delete("Organisation.MyDll", "1.0.1-branch.build", packageSource => _Pat, packageSource => true, false, _Logger);

The .Delete doesn't throw an exception or log any error, but the package remains in AzureDevOps.

I think I need to add .Wait() because of the async nonsense, but even so: the packages appear srikethrough in the Azure DevOps website, but not in the feed's recycle bin, and they're still listed in NuGet package manager in VisualStudio.

How do you actually delete the things?

Update

So apparently the C# only delists and you have to use the HTTP API directly to actually delete. However, it's impossible to authenticate.

        private static void Delete(NuGet.Packaging.Core.PackageIdentity packageIdentity)
        {
            // https://docs.microsoft.com/en-us/nuget/api/package-publish-resource
            HttpClient httpClient = new HttpClient();
            httpClient.DefaultRequestHeaders.Add("X-NuGet-ApiKey", "VSTS");
            //httpClient.DefaultRequestHeaders.Add("X-NuGet-ApiKey", _Pat);
            //httpClient.DefaultRequestHeaders.Add("X-NuGet-ApiKey", "[email protected]:" + _Pat);
            httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", ":" + _Pat);
            //httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", "[email protected]:" + _Pat);
            //httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer",  _Pat);

            // https://docs.microsoft.com/en-us/rest/api/azure/devops/artifactspackagetypes/nuget/delete-package-version?view=azure-devops-rest-6.0
            HttpResponseMessage rx = httpClient.DeleteAsync($"https://pkgs.dev.azure.com/Organisation/_apis/packaging/feeds/FeedName/nuget/packages/{packageIdentity.Id}/versions/{packageIdentity.Version}?api-version=6.0-preview.1").Result;
        }