I may be wrong, but I believe dataminers found the code for literally all dexited Pokemon (including megas) in the SV code, and they have their actual stats, movepools, and whatnot all coded in. This is different to Gen 8, where they technically exist, but are not usable. This has caused quite the major shakeup in the Balanced Hackmons community as well. Basically, when a dexited mon is sent out in a wifi battle, it looks like a Pikachu, but behaves like the dexiteed mon, retaining all stats, typing, and everything. So, in theory, I believe it is possible to "use" megas in SV (whether it is ethical or not is a whole other question, as hacking is definitely necessary), but I believe you can't send a Mewtwo into battle and mega it as is.
Hope that gave you some new info!