Furthermore, always scan downloads from any source—even Archive.org has seen malicious uploads—and consider supporting official re-releases via Nintendo Switch Online or the NES Classic Edition.
In practice, Archive.org plays a careful game. You will find complete collections, but if Nintendo issues a specific takedown for a specific title, the Archive complies. The result is a constantly shifting digital attic: some shelves are full, others have ghostly gaps where Donkey Kong used to be. nes roms archive.org
Before you rush off to download the "NES Games (TOSEC)" collection, remember the ethics of preservation: if you own a physical copy of a game, downloading a ROM is generally considered legal fair use (at least in the preservation argument). If you own nothing and download 800 ROMs, you are technically infringing copyright. The result is a constantly shifting digital attic: