That mechanism has been in Steem for quite a while, at least a few hardforks. But there's been no UI update to steemit.com to make use of it. Interestingly from what I've seen it's not a binary option but a variable to cap post rewards at an arbitrary number.
Currently all posts made with steemit.com are capped at a $1,000,000 post rewards. So limiting! :P
Ah yes, I remember seeing that number in steemd. I was thinking of the "Decline Voting Rights" feature.