Psychic on Gengar?

What YouTube crackpot or click bait article told you shadow ball was going legacy?

Do it. It's very very unlikely that ShB will be legacy, but do it now in case Niantic decides to ruin the game once more.

The whole point of Psychic is to guarantee people will raid more, for TMs to change to SB.
They won't ruin well planned bait by removing the tasty bit.