The end of the Nintendo Switch 2 reveal trailer answers one bigger question right away: yes, it will support Nintendo Switch games, both physically and digitally. That's what you would expect from such a successor, since Nintendo has a long history of offering direct backwards compatibility in some form. The Wii U effectively came with a built-in Wii, for example. In fact, the Nintendo Switch was the first Nintendo console in 16 years and the first Nintendo handheld ever to not support games from the previous systems, which was mainly due to the different format. But after successfully building this new brand, another full reset of their customers' libraries would have been questionable at best. People have big Switch collections and want to carry them over, naturally.
However, Nintendo notes that some games may not be fully compatible or supported at all, which is curious. One reason might be the lack of certain hardware features, where the infrared depth sensor from the right Joy-Con comes to mind, which seemingly got cut. And you can't blame them for leaving this out, since only very few titles actually made use of it, like 1-2-Switch, Ring Fit Adventure, or the Nintendo LABO stuff.
But this also raises the question of hardware compatibility. That is arguably also quite important and should be easier than ever in times of Bluetooth. In the very least I still expect my Pro Controllers to work, as well as most of the Retro Controllers (SNES, N64, SEGA Mega Drive), where all of them can be connected via USB. You won't be able to connect the classic Joy-Cons any longer or the NES controllers, at least not physically, but maybe you can still connect them via Bluetooth. In which case you could technically still play the games in question, just not out of the box.
It's also safe to assume that Nintendo will carry over the Nintendo Switch Online gaming libraries with all games still present... You don't want that Virtual Console reset for a fourth time. And on stronger hardware they can expand them with GameCube and Wii titles in the future, potentially even Nintendo DS and 3DS, since they could use the Joy-Con mouse functionality for touch inputs. Wii U probably can't be emulated just yet, but that's not required anyway, since almost all Wii U games got ported over to the Nintendo Switch already. But there is the prospect that the Nintendo Switch 2 might offer all of Nintendo's history combined at some point... Which would be delightful.