It's sad and happy to deprecate something that's no longer needed.

A couple of years ago, I built a tool called UWP Design-Time Data (GitHub | NuGet)

It attempted to provide some functionality that was available in Xamarin.Forms (XF) but not in UWP.
I saw someone from Microsoft show the functionality available to XF developers and immediately wondered two things? When will that be available to UWP? And, how can I recreate that until then? 

Based on the limited XAML extensibility of Visual Studio, I did the best I could. It wasn't perfect, but it was better than nothing.

Mine was never meant to be a permanent solution.
I always expected and hoped for it to go away. The last time I gave a conference talk about XAML tooling (in Feb 2020), I even talked about mine being a temporary solution.

Version 16.7 of Visual Studio (actually released in the middle of last year) added this as part of several other XAML-related improvements.

Today I finally got round to updating the GitHub repo and Nuget listings.

I'm not going to take them down, but they're essentially deprecated, and I don't ever expect to update them again.

It's sad to think that this is the end of this project.
All the time and effort I spent on this has no future value.
Hopefully, at least some of what I learned in the creation and support of this will be useful in the future.
But, it's good to know that this small but useful functionality is now available to all VS users, not just those who hear about and install my extension.
It's also good that to think that I was able to help some people for a while.

Today is a time to celebrate what was and look again at what comes next. 🥂