I read somewhere you can Hero's Downfall in response to Sarkhan's +1 as the indestructibility goes on the stack. Is this right? You can't downfall a planeswalker without them being to activate it so how downfall kill it?
Yes, you can do that. Sarkhan's +1 goes on the stack, and he doesn't gain indestructible until it resolves, so you can respond with Hero's Downfall.
You can't downfall a planeswalker without them being to activate it so how downfall kill it?
You're referring to the fact that the active player gets priority after a planeswalker spell resolves, which means that they can activate one of its abilities. The case with Sarkhan is no different; his controller is still activating the +1 ability before you have the chance to kill it. But while other planeswalkers' abilities will usually still be beneficial when they resolve, Sarkhan's doesn't do anything if he's already dead.
Sarkhan, the Dragonspeaker
Yes, you can do that. Sarkhan's +1 goes on the stack, and he doesn't gain indestructible until it resolves, so you can respond with Hero's Downfall.
You're referring to the fact that the active player gets priority after a planeswalker spell resolves, which means that they can activate one of its abilities. The case with Sarkhan is no different; his controller is still activating the +1 ability before you have the chance to kill it. But while other planeswalkers' abilities will usually still be beneficial when they resolve, Sarkhan's doesn't do anything if he's already dead.