Probably?
If they do it should be part of the paid online service and not its own thing.
I've never used it and probably won't this time. I felt there were too many Pokemon across too much hardware with gen 3. Now that we're in gen 8 the 'gotta catch em all' side of Pokemon has died a death.
However I did see a lot of pokemon from a lot of gens in S&S so maybe, with the power of the Switch most if not all, across both games, will be there?