RFC 7997 describes the policies about where non-ASCII Unicode characters can appear in RFCs. It was last updated in 2016.
As we have gained more experience with RFCs, and now that the canonical format for RFCs is no longer limited to ASCII characters, we will update this document to reflect the actual practice and clarify ambiguities.
This is an IETF document, so the IETF Note Well applies to contributions.