I think it's safe to say that - it's mandated by the IETF in
RFC 2277 - IETF Policy on Character Sets and Languages which says:
Protocols MUST be able to use the UTF-8 charset, which consists ofthe ISO 10646 coded character set combined with the UTF-8 character encoding scheme, as defined in [
10646] Annex R (published in Amendment 2), for all text.
RFCs don't use the word "MUST" unless they really mean it!
In case you're wondering: UTF-8 is the version of Unicode that uses standard ANSI 7-bit characters if it can, only going to Unicode for "foreign" characters. So, it's the best choice for documents that are mostly English characters, with only the occasional symbol or odd character, and files don't double in size if you use it. For other languages you'd use the relevant Unicode encoding.