Going on a slight tangent but the branches at the end of Loki S1 were created between the End of Time and the Beginning of Time a-new. There is no year for that event because it is within a void between time. Now normally time is circular and repeats infinitely, unless something within that threshold occurs that disrupts the timeline. Anything that branches from that point is retroactively concurrent with the main timeline at that point.
And the villains in No Way Home were basically plucked from specific moments in the past due to Dr. Strange's botched spell(?).
On the other hand, for Across the Spider-Verse,
It is intentionally not set in 2023 or later because of "canon events". Those are events from the past set in stone that no one from another universe can interfere with because it would create a paradox that ultimately destroys that timeline they changed.
HOWEVER, this is different than someone within that same universe time-traveling and causing changes that create a branch. (ex. Coulson's team in Agents of SHIELD season 7 went to their universes past, but later in the season caused a branch when too many changes occurred.) Since it's not a different universe until the individual of that home-universe causes changes to the home-universe's past.
Basically, if you're from some other completely different universe and you create timeline changes, you've corrupted the balance of the timeline by bringing outside elements/forces into the past and creating changes. This upsets the natural order and it obliterates everything - timeline and universe.
This is also known as an Incursion.
^ said spoilers MIGHT be tied to Loki, but that might be what the TVA is attempting to prevent (and maybe what Kang wants). Resetting the timeline before it can become a branch as opposed to changing it after the fact and thus obliterating an entire universe instead of essentially 'rewinding' it and arresting the individual who cased the change.