That's pretty interesting. I have a Mew in Shield and I haven't gotten a dex entry for it; my dex is still at 400.That's pretty unusual, they all (except Slowpoke, seemingly?) have dex entries, even their gmax forms.
For example here's Mew's
Sword: It's very intelligent and can use an incredible variety of moves. Many believe that all other Pokémon are descendants of this one.
Shield: This mythical Pokémon is said to be extinct, but sightings of it are still being reported to this day.
These Mew's first brand new entries since Gen 3, even!
Guess they'll be used for the Armor & Crown dexes, at least