When marking one Route to be Obsolete, OpenAPI swagger shows all routes are obsolete.

[Route("/api/Items/{Id}/Delete", "Delete"), Obsolete]
[Route("/api/Items/{Id}", "Delete")]
public class DeleteItemRequest : IReturn<ItemsResponse>
{
    public long Id { get; set; }
    public string Name { get; set; }
}

This declaration marks DeleteItemRequest with [Obsolete] attribute it does not bind [Obsolete] attribute to particular route, it's equal to

[Route("/api/Items/{Id}/Delete", "Delete")]
[Obsolete]
[Route("/api/Items/{Id}", "Delete")]
public class DeleteItemRequest : IReturn<ItemsResponse>
{
    public long Id { get; set; }
    public string Name { get; set; }
}

If you want to mark operation as Obsolete only for particular route, you should use ApiDeclarationFilter to set Deprecated property to true for operation Delete in the paths with key "/api/Items/{Id}/Delete"

Plugins.Add(new OpenApiFeature
{
    ApiDeclarationFilter = (api) => {
         api.Paths["/api/Items/{Id}/Delete"].Delete.Deprecated = true;
    }
});

That'll work. It'll be more handy if there is a way to do it within the Route declaration. Maybe something like [Route("/api/Items/{Id}/Delete", "Delete", Deprecated = true)]