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?
I think it will be avaliable for Sword & Shield, the whole Point of Pokémon Bank was to be able to store Pokémon from as far a gen 3 and be able to use them in later generations for nostalgia purposes. Not only that but there are bound to be unobtainable Pokémon in these games where transfering Pokémon will be essential.
Though I do think they will need to make a Switch version of Pokémon Bank for people to link their 3DS version so that they can make easy transfers.