User blog comment:MakeShift/The Crisis and its impact on the Arrowverse Wiki/@comment-31524712-20200124154231/@comment-2054358-20200129224132

Okay, so that's another reason not to merge Lucifer. If the cameos on Crisis don't effect their shows, then they are not part of the Arrowverse. They just have non-canon Arrowverse counterparts. If the Birds of Prey series comes back after twenty years, Helena's not going to remember the Anti-Matter wave that destroyed New Gotham because the changed timeline of the entire multiverse undid the events of Crisis in the non-Arrowverse shows. And not every show has a telepath like J'onn J'onnz to restore memories, and even if they did, they probably wouldn't want to reference how someone else's show changed them and J'onn J'onnz of Earth-38 is conveniently the only telepath to be a paragon, so the other telepaths in the multiverse would be unaware of what happened to their Earths and wouldn't think to restore their friends' memories. Our Earth could have been wiped out and restored by the events of Crisis. It might be crazy to think that a show actually wiped out our reality but a Marvel writer confirmed that our universe, Earth-1218, was destroyed in an incursion when Marvel Comics did their own Crisis that merged the main Earth-616 Marvel Universe with the Earth-1610 Ultimate Marvel Universe and we were brought back when the event ended. But obviously we don't remember that because it didn't happen. Or did it happen and we're just living in a new timeline with false memories? That can explain the Mendala effect.