Email clients don’t really have a spec to follow other than parsing messages that are received via POP or more likely IMAP these days (not forgetting MAPI either). Adding FWD or RE to a subject field doesn’t appear anywhere in the email message RFCs, starting with 822. As such they are undocumented conventions or extensions to the specs, and as the article points out technically irrelevant - no MDAs, MTAs or MUAs that do not “support” features that utilise these conventions will fail to parse a message.