I doubt that anyone will care about point of departure. Rather, if there are regional differentials, they will most likely be based on whether one has been in or around Seattle within the past 14+ days.
Thus, the issue won't be SEA departures, but where you've been, even if you are departing from elsewhere.
None of that has anything to do with the possibility of domestic travel restrictions.
Not worth worrying about until and unless it happens as there are always permutations and until one knows what those are, there is nothing to be done.