A patch was released today that will affect speedruns. In some cases, the old version will be faster (Chiyari cheese is a lot harder now, and I've heard Suika's spell can no longer be quick-killed, for example). How should this be handled? In my opinion, a custom variable should be used for old versions and they should be hidden by default or given a separate leaderboard.

Yes, I thought about this one. I will add another variable to every category and make it the main one. I don't think I should hide it, but it would be unfair if they were in the same leaderboard, due to the difference in killing some of the bosses(and gameplay as Chiyari). I just wasn't fast enough to do it before this post

Would it be possible to still submit runs for the previous version of the game if we still have the files saved? I have both the Ver. 1.00a and Ver. 1.10c saved (was able to follow instructions online on how to redownload and reimplement the game's previous version through Steam's console), so doing runs from both versions are possible for me.

Yes, it would be possible, that's why before 1.10c isn't archived, but it might take a little more time to verify runs to see, which version is it, due to the lack of options to just see the version itself