I have been wondering about this for a while. Most runs are 30fps on YouTube, but some are below that. Recently I got a run at 15fps, and was wondering at what point does it become a problem with low fps for moderators to accept the run.

That depends on the game. I think there are some games where you can pull off some tricks only with high fps. So for these runs there might be special rules to verify these tricks. I know of some leaderboards which require 60fps recordings. This might be due to the fact that turbo is allowed. On the other hand, if there is nothing preventing you from verifying the run to be on e.g. 15fps then that should be fine. If you don't feel comfortable with that speak to the runner and make them aware that the settings are non-optimal. Maybe you can come to an agreement.

I’ve verified a 12fps run before. It just depends on the game

I've had runs as low as 6 FPS verified, if I'm not mistaken.

I personally go as low as 12 fps, but its definitely game dependent

I'm sorry but I'm not gonna verify a slideshow presentation, way too difficult to know if the run is legitimate

as long as i can see whats happening and when the run starts/ends is clear its fine

It depends on wether or not fps affect the games mechanics