i'm assuming that a cap above 60 isn't allowed because of technical instability? does the rule have room to be changed as issues are (potentially) fixed?

While high FPS is interesting for labbing, the developers have stated they plan to change its behavior in the future. To keep things simple, we've decided to allow only 30 and 60 FPS for now. It might change in the future

I've played in 144FPS casually and I haven't encountered a single bug. It would make sense to limit it to 60 and 30 until a new patch comes out.