where is downpatching allowed? is it any category or only categories where the wr is only beatable with an older version?

Down patching is allowed for every level run, multi-level runs are a bit more complex but the limits for those categories are just that you cant down patch to a version that does not have required levels for the run (For Ex: Currently fullgame categories can go to the day 1 patch of Violence but they cannot go to a version that existed without Violence.)

ok, thank you!

Also downpatching to use strats that aren't used even in a WR is frowned upon. Eg using the overpowered nukes that were patched last patch is well, maybe not forbidden by rules but we have a gentlemen's agreement to not use them.