Seeing a few reports on Reddit of YouTube breaking after add-ons come back with 66.0.4.
"I updated and all add ons are back, but now my adblocker makes all youtube videos error and black screen for like 20 seconds before the video loads." Link)
"pdating to 66.04 from 66.03 has caused all YouTube videos to fail to load. Just a nice spinning grey circle in the middle of a black screen." Link)
I've asked for further clarification in the threads.
The second user came back and reported that disabling "Block websites from automatically playing sound" fixed the issue, but those symptoms don't match what I would expect from audible autoplay blocking's normal functioning.