1) Go string type is specified as a sequence of bytes. Full stop. Any encoding, binary blob, string does not care. Particular stdlib string functions _do_ care, expect UTF-8 encoding and document the fact. Other stdlib string function work with the sequence as a sequence.

3) Go specification requires UTF-8 in only one place: the encoding of Go source files string literals.

2) Pure ASCII is a subset of UTF-8, ie. pure ASCII _is_ valid UTF-8. There is _no_ more work and there are _no_ security problems when working with pure ASCII as UTF-8, because that's what it is.