Could this mean legacy moves come back?

Spaghetti programming ftw.

I doubt we'll see legacy moves intentionally come back in this manner. Some of those moves went legacy due to main series incompatibility issues and who knows why other moves go legacy?

Depending on how PvP shakes up Mewtwo may still be potent without Shadow Ball. In main series competitive formats Thunderbolt and Ice Beam (referred to as "bolt-beam") lets a Pokemon hit literally any type for at least neutral damage. Mew and Mewtwo are the only two Pokemon in PoGo that have access to both of these moves and the ability to unlock a 2nd permanent charge move will let them wield both simultaneously. Obviously Shadow Ball is superior to both of them but now people that missed out on Shadow Ball have a solid alternative for PvP.

I think it's going to depend on where those Charge Moves are usable. If, for example, you have 1 Charge Move for Raid Battles and Gym Battles, and the 2nd Charge Move is only available for PvP battles, they might just re-add it to the 2nd Charge Move possible move-pool.

But if both Charge Moves are available at all times (which... might be simpler for Niantic to do) I'm thinking that we are going to see a way for Community Day and Special Event (EX Mewtwo, Lick/Psychic Gengar, etc.) moves to be "Trained" for, but I think some of the other Legacy moves are essentially gone forever. Double-Rock Omastar? Gone forever - get Trading with someone who quit playing in 2016 and still has one! Smackdown Tyranitar? Trainable.

I don't see the reasoning behind bringing back legacy moves for this update. I can't think of a single mon that doesn't have at least 3 moves so there's no reason in that sense for Niantic to expand any mons movepool. How useful it will be to have that second move...