Rayquaza mega evolves without a megastone, and It learns Dragon Ascent in Sword and Shield, so I figured It should be able to, but there are no mega evolutions in Sword and Shield.
No, it can't
Mega Evolution does not exist in Pokemon Sword and Shield because both Megas and Max Pokemons would make Gen 8 too powerful.
Although you can get Rayquaza from Dynamax Adventures, you can't Mega Evolve it, sorry.
Hope this helped!
Source
No, Mega Evolutions in general don’t exist in SwSh, so Rayquaza can’t mega evolve.
