I think it would be worthwhile to mention a couple
reasons, when the decision was made to use Roundup, etc. Without it, a
casual reader might think the core devs made a horrible mistake way
back when, and are only now getting around to correcting it.
I was not involved in core Python development back then, so if it is really important and if people think such paragraph needs to be part of the PEP, then perhaps someone else more knowledgeable will need to help with this.
Personally, I don't think it was a horrible mistake. I believe the core devs back then carefully considered all options and decided that bpo/roundup was the way to go. And I really don't want to give that impression to the readers of this PEP that "I" or "core devs" now think it was a horrible mistake. If there is specific parts of the PEP that gives people that impression, then I'd definitely want to work and improve that.