I use VSC for all my C# work on my Mac, and noticed that the quick-fix menu had stopped working properly. It used to suggest Using statements, or even prefixes to add to objects that came from another namespace.

I finally found a post that came up with a fix:

https://developercommunity.visualstudio.com/content/problem/249423/using-imports-no-longer-suggested-in-the-quick-fix.html

So, I needed to enable Source Analysis under Text Editor. Now it works as before!