The third split with the failed hero timeline definitely came out of nowhere, but the original split of child and adult timeline was 100% established well before they released Hyrule Historia. It is directly brought up in the story of The Wind Waker if you just play the game. Remember, after OoT we had MM which was clearly following the events of child Link after he returned to the past, while even the ending credits of Ocarina clearly depicted there being a separate adult timeline in which the people of Hyrule celebrated the defeat of Ganon and the Sages watched over them.
So, when Wind Waker came out after MM, it states right from the beginning the events of Ocarina of Time and Link's victory over Ganon, which only happened in the adult timeline. So, by Wind Waker's release, we canonically had two games that each followed a different timeline, both of which had been depicted in the end of Ocarina of Time.
This was not constructed by fans or to appease to them. This was knowingly done by Nintendo. Now, I would argue that most likely, it was done more out of convenience than in an attempt to actually deepen their lore or such; but that does not change the fact that both logically inevitable timelines were introduced, acknowledged and used by Nintendo, which in turn fired up fan theories and speculation - not the other way around.
This is where you are right.
This is where you are wrong. It doesn't matter if Nintendo had the entire timeline planned out from the very start - which they obviously didn't, because how would you even plan out a timeline for games and stories you have not yet even created. But to state that they don't care about their timeline at all is just plain wrong and there is plenty of evidence for that.
First of all, they always made connection efforts. Always. The first two games were connected, and ALttP, effectively rebooting the series, was declared a prequel so that it would make some kind of sense. Then, Ocarina of Time was effectively a remake of ALttP and a reboot in a sense, and in order for that to not directly contradict the other games, they presented it as a prequel again because it fit the descriptions of ALttP's intro's sealing war. It was also an origin story for Link's iconic look.
In the following games, they cared about their latest achievements with OoT more than with what came before it, so they concentrated on that groundwork and treated it as a clean slate and made the stories afterwards work in context to it. Most if not all of the following games had a kind of clear understanding of how they related to the events of Ocarina of Time as an origin point and worked with that.
Finally, with Hyrule Historia, they published their official timeline and introduced a third timeline split in order to get rid of the "inconvenient" games that had no such clear placement or which' plots were too ambigous to work for the overarching story, so they shoved all of that into their separate realm as to not having to inconvenience themselves with these games and their narratives any longer, but still have the freedom to draw from them should they ever need to.
Now, with Breath of the Wild, they pretty much reset things again to start off of a clean slate, similar to how things started over with Ocarina of Time. But still, they didn't say "This is a new Zelda game, everything before doesn't matter", but just pushed it 10.000 years into the future and were done with it. It was convenient and they consciously left it ambigous how the game is placed in the timeline, because they don't want to feel limited in what they can do, storywise, with the new games. And now, BOTW2 is in the works, and yet again, it's clearly a sequel to BOTW. So, they always made connections as far as they served their goals with the narrative, but decided to not shackle themselves to having everything being a precisely placed tile in a narrative mosaic. But they do care enough about connections and timelines to put effort into connecting the games somewhat, and I fail to see why so many people seem to struggle with admitting that. The whole "Nintendo had no timeline plans whatsoever and pulls everything out of their ass each time they make a game" crowd is much more annoying imho than the timeliner and theorizer pack of fans, because they just seem ignorant of both things happening outside and inside of the games.
