We previously had "3-win" category per characters, which would do what you're asking for and was the entire reason we asked for it in the first place, but it seems like that was removed.
Can confirm, just had the same thing happen with another leaderboard (https://www.speedrun.com/Jumps)
Hey, I added this as a new category, I just now decided to check this randomly.
Come to the Official Slay the Spire Discord to chat with other runners and ask for advice!
The most optimal solution would be for the Devs to make a Speedrun channel in the official discord. If they won't, then would make sense to make one.
My suggestion is that the runs that aren't on fast mode and the runs that are on fast mode be combined into single "Ironclad Any%" and "The Silent Any%" categories, the slow mode runs by runners that have fast mode faster runs become obsolete, and those that don't are just left. Hell, we could even add a column to the table indicating "Slow Mode" or "Fast Mode". It's just silly to have the two be separate, since slow mode is effectivly obsolete, and thus won't be ran any more, or at least, there's no logical reason to run Slow Mode anymore.
Edit: I guess that reply was more intended to be a response to Jimquisitive. Oops
I'm honestly somewhat confused why we even bother separating Fast Mode and not Fast Mode. In theory, going forward, why even bother running Not Fast Mode? The only difference between the two is that it's slower. I get that it sucks for those who established records before Fast Mode existed, as their records are for the most part going to be overridden by new Fast Mode records, but that's the inherent nature of speedrunning Early Access or Betas of games. I'm for condensing based on character Any%, regardless of fast mode vs slow mode, and regardless of fast mode V1 or V2.