I believe I've seen this with a couple other repos in the past monthish - for example, repos that have had their github version archived in favor of a codeberg or gitlab version that's linked in the readme - though I'm struggling to find examples of this from digging through my desktop browser history.
For Dnspy in particular, I think that https://github.com/dnSpyEx/dnSpy should absolutely be the #1 result - 5yr old archived repo vs an actively maintained community org is pretty straightforward here