Sarkhan the Masterless has the ability:
+1: Until end of turn, each planeswalker you control becomes a 4/4 red Dragon creature and gains flying.
There is a ruling associated with this Sarkhan that says:
Once Sarkhan’s first loyalty ability has resolved, each planeswalker you control (including Sarkhan) is no longer a planeswalker for the rest of the turn. They don’t lose any loyalty counters or abilities, and you can still activate their loyalty abilities if you haven’t done so yet this turn. They don’t lose loyalty if they’re dealt damage while they’re not planeswalkers.
Question: I was under the impression that rulings are all derivable from the comprehensive rules, but I can find no rules reference to "back up" this ruling.
The ruling makes sense to me, if a planeswalker is a creature, you don't want it to start losing loyalty counters for being in combat, but the above ruling makes it seem that Sarkhan has removed the "planeswalker" type from the planeswalkers. Which comprehensive rule supports that statement? I was unable to find it.
Note: This question is similar to: What are the ramifications of turning a planeswalker into a creature?, but it is different. In that question, it appears the creatures maintain their planeswalker status, where as Sarkhan removes it. So, ideally, an answer could address why in that context planeswalkers remain planeswalkers, while Sarkhan removes planeswalker status.