There is a lot of old stuff, especially on the wiki. It's one of the reasons I abandoned over a decade of config and started using Doom Emacs. I appreciate the curated set of packages with a more consistent interface than I had time to maintain.
As for sln based projects, I've never had success with them in Emacs. Heck, even VS Code has been hit and miss for me. Maybe someone knows a good sln package? The one you mentioned might be worth a try.