Why is Omastar not updated with the new movesets?

They should still be there, just that the current best moveset should always be A. If the legacy ones are better, they can be A+ or even S.

Because there are people who actually have them, personally I don't have any legacy omastar, but I have some other legacy mons, and knowing that SC gengar is better than Hex gengar helps, the only pointless legacy moves listed are those of the legendary pokemons, but they are not ranked, since nobody can have a thundershock zapdos

The best moveset is the best moveset, be it legacy or not. It get's an A. The current system is just fine, because it gives a complete overview, showing you that the current best moveset is inferior to the legacy best moveset. And there are people (like myself) who have a lot of legacy movesets and that are interested in all the information. Others can still see what the current best is. It works well for everybody.