The 3DS had backward compatibility with physical DS games. The Wii U had backward compatibility with Wii games (the Wii could also play GameCube games). No surprises here - especially if they are going to call it “Switch 2” I’d have been surprised if it wasn’t compatible.
3DS cartridges had an extra notch on them to stop them being inserted into a DS. Probably we’ll get something similar here or there will be some software mechanism to stop attempts to run the new games on the old system.