Andrew wrote (2022-01-19):

 JC>> I think in hindsight it might have been better for FTS to leave it
 JC>> as \## since legacy mailers used it and could not be updated, but
 JC>> it is what it is!

 AL> BinkD (the original implementation of BinkP) has always used \x## as far
 AL> as I know.

That is my understanding too and that it is what I was trying to tell. Binkd always used \x## (according to older source codes). I believe \## was never meant to be a standard, but it was a small (but significant) mistake in the documentation (FSP-1011) of the protocol. Then some authors implemented that erroneous spec (which was still a proposal) in their mailers. For some reason 4 years passed until a new proposal by new authors (FSP-1018) was published by the FTSC and another 2 years until it became a standard (FTS-1026). Meanwhile, since 2004, the Wikipedia page on binkp [1] linked to the original FSP-1011 [2] until I edited the page yesterday and changed it to FTS-1026.

So it's not that every mailer used \## before FSP-1026 and then deprecated it and changed the escape sequence to \x##.

Fun fact: FSP-1011 was written by the author of Binkd (Dima Maloff) and the author of Argus (Maxim Masiutin). Binkd used \x## and Argus \##. If they were confused and unable to document the correct escape sequence, no wonder that others made the mistake too.

[1] https://en.wikipedia.org/wiki/Binkp
[2] https://www.ritlabs.com/binkp/

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