https://wccftech.com/nintendo-switch-online-custom-nes-games/

The Nintendo Switch Online has finally launched and with it the ability to enjoy a selection of NES games with additional features and online multiplayer. Nintendo plans to release more games in the future, and it seems like there will be the possibility to add more games to the list of available titles through unofficial means.

Retro Engine and Splatoon 2 modder kapu recently revealed that it might be possible in the future to add custom NES games complete with online multiplayer. The Switch NES games are plain .nes files and the GUI seems to be like the NES and SNES Classic one.