Less than 24 hours after its launch, Nintendo's Switch Online service has already been hacked to allow gamers to load their own custom NES ROMs in to the NES library. The process, which seems to be simple, is possible because of Nintendo's use of common .nes ROM files, the same filetype used by ROM sites online.
https://www.notebookcheck.net/You-can-now-play-your-own-NES-ROMs-on-Nintendo-Switch-Online-thanks-to-a-new-hack.333420.0.html