Why can't Tasker manage battery mode on my rooted Pixel?
As it was confirmed in the comments, Tasker was denied superuser access in the Magisk app. Tasker can enable battery saver. 

If anyone wants a command-line solution, try this:

Code -> Run Shell -> Command: settings put global low_power 1 -> tick Root.

Replace 1 with 0 to disable battery saver via command-line. 

Tested on OnePlus 6 running Android 8.1.