Allowing Unicode characters, then stating best practice is to stick with ASCII, is weird. (Go is not alone in this practice.) Unicode identifiers have a host of issues, such as some characters have no case distinction, some have title-case but not uppercase, some "capitalize" the last letter in a word and not the first (Hebrew has five "final form" letters), etc. Does Go specify the meaning (exported or not) if a letter has no case, or if an identifier starts with a zero-width joiner character? Without a huge list of detailled rules, too much is left to the implementation to decide. I prefer to stick with ASCII for names.

Fun fact: When printing with movable type began, printers would travel with large "type cases" containing the small wood or metal blocks with glyphs on them. The ones the used frequently were kept in the lower half of the case, in easy reach. That's where the terms "lowercase" and "uppercase" come from.