In this case, it’s the Fortnite world.
If Fortnite were a game in the 1990s-2000s, the data (map, world, characters) would be on a CD or DVD and the multiplayer server would also be included on the CD: the community runs its own servers. If the developers release a huge new update - including over digital distribution - users still have the original discs and server software, thus if they want to relive “Fortnite 1997, v1.0’ they can - just reinstall it from the original media.
With the iOS App Store we used to be able to make versioned backups of the IPA files and restore them using desktop iTunes so if an over-the-air update for... say, Angry Birds, added an obnoxious amount of pay-to-win functionality then we had the choice to downgrade before things went to shit.
Now, we can’t do that. This is why I don’t buy mobile games anymore: I have no guarantees about my ability to keep what I paid for.