So what language would you chose then? Java, PHP, JavaScript? None of the big languages where perfect from day one and it does not really matter, since day one is over already.
I think this is a good thing. The styles are just opinions anyway and forcing everyone to just follow a single style takes a lot of bikeshedding away, which I really like.
Worse than not having a unused variable check at all? Dunno, the underscore assignment are very visible for me and stand out on every code read and review.
Honestly, it does not happen often that I have a ln unused variable that I want to keep. In my mind it is the same thing when wanting to call a function that does not exists. Also my editor is highlighting error Long before I try to compile, so this is fine too for me.
Yes it is a hard error and Go does not compile then. You can do _ = foobar to fake variable usage. I think this is okay for testing purposes.