Reply to what was sent as HTML, but with the original deleted, and the plain text option for Groups.io in force again.
Let's see what this does as far as the message footer is concerned. I suspect that HTML is going to be maintained because the message being replied to was HTML, even though all its content was excised from the reply before the first character of the reply was typed.
Brian *-* Windows 10 Home, 64-Bit, Version 1903, Build 18362
*A learning experience is one of those things that say, "You know that thing you just did? Don't do that."*
~ Douglas Adams (1952-2001)