I'm very new to speedrunning, so I'm sorry if this is a dumb question. I just installed SpeedRunIGT through CurseForge, and when I open up the minecraft launcher(with SpeedRunIGT), it says "fabric-loader" under the "installations" dropdown. Is this okay? I know the rules say that "Fabric API" isn't allowed, but what about "fabric-loader"?

The fabric loader is allowed. In fact, a good majority of speedrunners use it to run minecraft with a several mods like CaffeineMC mods, LazyDFU, atum, ect. Those mods do not require the Fabric API and are not banned because of that.

Actually, let me rephrase that last sentence. The ability to work without Fabric API is just one of the reasons the aforementioned mods are allowed, there are plenty of other reasons too.

Fabric Loader is the way that most speedrunners do it. In fact, I'm pretty sure forge isn't allowed for speedrunning.

Thank you!