I have the version when greed came out saved in my computer and I can still do things like filth clip and stuff like that But I don't know if that's allowed in runs I also have the october 9b patch and the Saw Your Heart before they nerfed the alt nailgun

Yes its allowed as long as when you submit the run you say what patch its from, greed patch was 6/23/21 or somethin like that iirc

but is it allowed to downpatch through patches of a single listed version? (i.e, downpatching to prenerf sawgun although both versions are listed as one in the version list)

I'd assume so, I did it for my P-1 P% run iirc and that was verified.

yeah i used one for my new 2-2 run so i guess

It is fine to use older versions of the game just keep in mind that archived boards are no longer open because I have them archived on my end, and when the game comes out of EA all runs done on EA and earlier will no longer be valid so it is recommended that you play on the current version of the game for ILs. Also when wrath comes out you will obviously have to update to do runs with that.