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.
youtube dl helper is pretty cool if you have lots of hard drive space and have videos you watch over and over (like ASMR, nature sounds, etc). just gank that shit without any ads.