Time Shenanigans Explanation · 12:31pm Apr 16th, 2022
So, as explained in UME, since DD is an anomaly, then them intervening on Sweetie Belle turned her into an anomaly. By extension, everything she influences, people and universes, become anomalies, just existing in the present, with no past and future. So this means that time travel is normally not possible unless it is to go to a time before the universe became an anomaly (so before there was a split in the timeline because of the intervention of an anomaly). And this means that, in universes where a modification in the past causes the future to be rewritten instead of causing a split, the whole timeline simply disappears excepted the present.
But there is one instance where it is still possible to time travel: when there is a reset. Like in Undertale. Frisk can still time travel because she erases the present and returns to an earlier present. Same for the events of Majora's Mask. Each time that Link played the Song of Time, the Goddess of Time reset the present to the dawn of the first day.
What about the time travel in Ocarina of Time? Well, it is kind of a special case. Through the Master Sword, a bridge existed between the present and the past, and as long as this bridge existed, the past was considered as part of the present. Then, when Zelda sent Link back in time at the end to stop Ganondorf before he could start his plan, she did a reset of the past.
I hope it all makes sense. Where is a Doctor when we need one to explain timey wimey stuff?
The Doctor never really explained anything, because Gallifreyan five-year-olds are smarter than we'll ever be and his attempts to dumb things down reflect that.