I don't know if there is a banned flag inside the NAND, and even if there was you still have the it is not the NAND but the servers that control things, and said servers can also be more complex than that (flagged for ban but not yet banned sort of thing).
On older consoles we have seen things like various support services, account logins, purchasing options, warranty services... are denied to banned users (such a thing invariably starts with you tapping a serial number, generating a debug code or doing something similar into a service/email and going from there) but I don't know if any have been discovered and are currently active/working for the Switch (certainly don't see any discussed like we did for said older consoles).
When people ask "Is there any information anywhere inside the nand that would tell me if my console was banned" that tends to be more for when they hacked a console, kept it offline throughout its being hacked and subsequent hacks, and are now looking to restore it to stock but hoping nothing slipped out in the meantime or sticks around deep inside the console for just such an event (else I might as well keep the hacks on there sort of thing). For the Switch then right now nobody has gone looking for easy to remove software flags inside the NAND image so it is mostly only an option if you have your clean NAND image backed up from before you did the hacks. As far as I am aware Nintendo has not been creative and tried to bury some kind of hardware flag deep within the system, or even some kind of TSR (you should do a full power down/hard reset anyway upon restoring to make sure it has taken properly).