What dificulty does TF2 need to be on to speedrun it? I'm new to speedrunning so there might be something I'm missing.

people usually run on the easy difficulty but you can run on whichever dificulty you want, just make sure to sumit to the master category if you want to run on that dificulty

All Main Categories may be run on any difficulty the runner wishes with the exception of any% Master which must be done on Master Difficulty. The General Recommendation however is easy since the routing is almost virtually the same between easy and master difficulty up until Trial by fire, but you have a much higher chance of randomly dying on normal and above to enemy NPCs killing you.