Me and my SO have been playing Breath of the Wild each on our accounts, on the same Nintendo Switch.
If one of us buys the DLC, is it accessible by both accounts on the console or only to the account that bought it?
According to the Nintendo help pages:
While a console is active for your Nintendo Account, everyone on the console can play the games you’ve purchased.
Although this is talking about games, I can't imagine that DLC would work differently. One Reddit seems to confirm that DLC works just like the games:
...I can confirm that the dlc works on both accounts.
Numerous articles, such as this IGN one on the internet state that purchases made on the Switch are linked to the account. But what that means is, if you were to move you account to a new console, you can download anything you've previously purchased at no cost again, something that doesn't appear to be possible on previous Nintendo platforms.