will shadow ball become legacy after gengar day?

Don't believe everything you read online. If there's no credible source, ignore it.

But yes, Gengar would stink without shadow ball, which is a good reason to believe that it won't go legacy.

Psychic is permanently replacing Shadow ball during this event. ;)

That would be the ultimate troll by Niantic. People have been asking for another chance to get SC or Lick to replace their eternaly inferior Hex Gengars so they can keep up with people lucky enough to have almost 2 year old Legacy Gengars. Now that time has come. Baiting people with the move and then Taking away Shadow Ball would be really cruel and a nasty thing to do.

There's absolutely nothing indicating they'd do that. While they don't always announce moves going legacy they did for Mewtwo, likely because they know how sought-after SB Mewtwo is. If for whatever reason SB Gengar went legacy I don't doubt they'd announce it beforehand.

Even so, why would Niantic actively kill off the ghost type? The only other powerful Shadow Ball users left would be Alakazam who's outclassed against psychic and ghost types by other ghost and dark type users or Darkrai, an unreleased mythical that would either be a one-per-user research or an EX boss.