It's official: isn't part of the formerly created timeline by Nintendo. Of course, this means that isn't on the timeline either, being a direct sequel to. Considering that neither game includes a lot of mentions of the events of other games in the series to help find their place on the timeline, these games being separated from that timeline makes the most sense compared to other possibilities, such as being at the start or end of the timeline.
The main theory previously for when took place was that it's so far into the future that the events of other games have become legends. While this wasn't a bad theory, it isn't as fitting as giving and a separate space on the timeline where they can be freed from the universe created by previous games. It's not a secret that the official timeline is messy and convoluted, especially with three branching timelines based around the events in, so it's good to have a bit of a fresh start for newer games.
It's possible that the games prior to weren't necessarily created with the idea in mind that they would be connected on the same timeline, but after enough demands from players, Nintendo eventually released the official timeline prior to. Now, there's a new timeline that's official, and it places — and by extension — on their own, separate timeline. This is the best choice that could've been made for those games, not just for clarity, but because this is the opportunity to start from a clean slate for the development team.
Tears of the Kingdom has one noticeable plothole as a direct sequel to Breath of the Wild, but there are a few easy ways to explain it.
Trying to design games with the entire timeline in mind and how the current project would fit among the rest of the games probably wouldn't be easy. Starting over with a new timeline takes away that potential burden, even if it wasn't part of the creative process to begin with, and it keeps players from continuously speculating about how they can piece the whole series together.
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