Apparently this is to resolve the performance issues the game has been having.
Fortnite has been having some performance issues on the Nintendo Switch. Epic Games promised that they are looking into them, and that they hope to resolve them with the next major update for the game that they release. True to their word, they did release a new update aimed at fixing the performance problems with the Switch version of the game.
One of the ways the game does that, apparently, is by blocking the Switch’s video recording functionality. The functionality works on a whitelist basis (in that games need to explicitly allow it, and it is not available by default), and is known to have some performance costs; presumably, Epic feels that disabling it leads to major gains in the game’s performance, or they wouldn’t have gone to the effort of blocking it.
It is unknown whether or not the change is permanent, and Epic themselves haven’t actually officially acknowledged it at all. Fortnite is available now on PS4, Xbox One, Nintendo Switch, PC, iOS, and Android.