I have read somewhere (banned users has also confirmed this) that even if your system is banned, Nintendo will still allow your system to be updated to the latest firmware. Even though you can't access any of Nintendo's online services, Nintendo still wants you to enjoy your Switch, as new games get released, they will require newer/latest firmware. Since your already banned, at lease you can still go out and purchase legit retail cart games that require higher firmware, however in saying that, these new games should have the latest updates on the cart anyways for you to update.
This is pure speculation on my part only and not by other sources, something tells me Nintendo can't force your Switch to stop receiving FW updates, whatever method they are using to try and stop online updates might be affecting offline cart updates as well, so they decided to let only FW update go through.
If Nintendo blocks updates completely, online and on carts, then your Switch will be a expensive paper weight pretty soon, so at least they are in some way doing the right thing.
Then that is when ChoiDuJourNX comes into the picture

Update FW manually regardless.