I'm not sure I agree with the sentiments, but yes I've read why it's not wanted in core or even listed as a contributed module. I've managed to work around it with editing templates files this far, but I'm currently stuck on something (I'm trying to port a site over that uses webform2pdf - but generates QR codes in that PDF from a separate library.) As far as I've been able to tell, there's no workaround for this particular case.
But that's a whole different problem that wouldn't need solving if I could get the PHP filter working. I've tried porting it over with coder-updgrade and some manual changes after that (admittedly not up my alley, but at least it's a small module.)
Unfortunately, the module seems to rely on making changes to the 'filter_format' directory which existed in Drupal 7, but not in Backdrop. I'm really not a big database guy, but I've done my best to try and find anything similar in Backdrop tables and I can't. I don't mind workarounds (and that'll be my next post) if it's futile, but I think the workaround is too based off what I've tried.
Surely someone has overcome this? There's surprisingly few posts and no resolution on them.
Any help very much appreciated, I have to get past this somehow.