if you're a uBlock Origin user (or use an adblocker with a similar cosmetics modifier), you can add these commands in the uBlock dashboard (under My Filters) to get rid of it!

youtube.com##+js(set, yt.config_.openPopupConfig.supportedPopups.adBlockMessageViewModel, false)

youtube.com##+js(set, Object.prototype.adBlocksFound, 0)

youtube.com##+js(set, ytplayer.config.args.raw_player_response.adPlacements, [])

youtube.com##+js(set, Object.prototype.hasAllowedInstreamAd, true)

This is coming from /r/pcmasterrace, so I don't know how accurate it is. Worth a shot if you want to keep using that site.