assuming that there is even a way to get to the past anymore. The season has already established the notion that the last of the time portals were destroyed, and while it may be possible for them to find an unknown additional one, the season is currently still working on the premice that the portals are gone.

well that would be if he traveled back in time and killed Aku. If he killed Aku in the future then the timeline would remain the same and Ashi would still erxist