Can't give a definitive answer but my allow/deny list parser has always used `MIME::QuotedPrint` through `html2text` to convert them from MIME'd HTML into plain text and that was last updated September 2021.

I've also got a test note from late 2020 which is also `text/html; quoted-printable` which suggests at least iOS 14 (I don't think the 15 alphas would have been out by then.)