Ah! I’ll put this here for now. It’s a v5 API, and I’ve been furiously working on those with the v5 announcement. 
v5 only
POST https://api.twitch.tv/kraken/channels/{channel_id}/bits/events/recent/dismiss
Required scope: chat_login
User must be broadcaster, channel mod, or message owner.
Returns a payload with info about the pinned cheer you just dismissed.
401 Unauthorized if the user isn’t authorized to dismiss the pinned cheer.