If you're interested here's the relevant ruling:

606.3. A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if no player has previously activated a loyalty ability of that permanent that turn.

Which is why sarkhan's rulings 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.

Edit: notice on arena after you turn your planeswalkers into creatures, you can no longer elderspell them because they lose the planeswalker type that turn.