I’m guessing it will be backwards compatible. The Wii, Wii U, DS, and 3DS all had backwards compatibility, and the only reason the Switch didn’t is because it has a vastly different architecture from the Wii U (and no space for a disk drive). I can’t imagine that the Switch 2’s architecture will be vastly different from the Switch, and they’ll both presumably use cartridges, so I’d say there’s a very good chance of backwards compatibility.