Are there truly people out there that are terminating HTTP/2 at the application level? That's really quite surprising for anything serving production traffic.
Depends on what you mean by Solargraph, because it's kind of a large project. If you mean their typechecking, then definitely not, because it's not using a Ruby parser. If you mean the general feedback, then yeah potentially.
Rubocop yes if it ends up using YARP as a new backend.
Either way, I would suggest you check out ruby-lsp, which is definitely going to benefit from this speed, and soon.
The templating is definitely an effort to keep the maintainability in check. It's also because we're planning on keeping a grammar around to generate test cases for us with fuzzing/other algorithms.
The encodings is a bit historical, to my knowledge Ruby is the only major language that was developed by someone whose language did not work with ASCII. So the first encodings written in Ruby were the old Windows pages.