Mew's been shiny locked in every event except for the Old Sea Chart in Japanese Emeralds. The event had a very limited distribution and was supposed to be locked. It wasn't by mistake. People didn't soft reset for it and RNG abusing didn't exist back then. If you find a shiny mew RNG'ed from that event, it was gotten by hacking in the event into the games years later, making it a hack. Any other shiny mew was created with an Action Replay, PokeSav or things of the like. So they are all banned from being traded.
Hope this helps!