someone could get lucky and find a master ball during the run.

True, I'll talk to the mods about this, cause Catching Eternatus is very much possible

wait, you can catch eternatus?

I think you can after you beat Classic for the first time, not 100% sure.. However for Classic% runs, catching Eternatus is banned. It removes a massive challenge and makes it to where everyone just start fishing for Master Balls to chuck at it on the 2nd phase.

You can catch eternatus after completing the pokedex (besides eternatus)