While we can assume that branching timelines are not big enough to qualify for tier 2 because; "Two universes A and B are spatio-temporally separate if and only if there are no points in space or time that are in both A and B" and under this definition, timelines that branch off of each other are not, by default, separate spacetimes. Such timelines clearly share not just a single point, but an
entire interval of time, that being the timeline that existed before the moment at which they diverged. So while branching timelines are not separate space-time continuums, there are instances where the branches are big enough to qualify as a tier 2 structure, if each branches extend infinitely; This would mean destruction of an entire branch that is not by some sort of time-paradox or casualty but destroyed directly, would qualify for tier 2 as;
Destruction of intersecting timelines qualifies for hier levels of tier 2, if and only if each timeline, without the parts that intersect with the other, is still of large 4-dimensional size. (Large in the sense of criteria "A)" in the
Tiering System description of Low 2-C)
This includes branching timelines, but leaves out things like a timeline which only branches out for 2 Planck seconds before fusing back together.
Given, what "large" means in a time sense is of course hard to tell. Roughly one could say that they have to be separate for infinite time.