> As far as I can tell, they never had support for finding usages across packages, never had support for finding implementations of an interface, never had support for finding what interfaces a certain type matches/implements. These are basic features in any IDE for a language that has packages and interfaces.

I suppose it depends on what you're used to. I've never found myself wanting to do these things, but I'm also strictly from a Python and C++ background so my standards are admittedly lower than Java and C# people. Hopefully gopls solves for these problems.