image is a tweet from the AO3_Status Twitter account, posted Jun 23, 2021
Starting July 23, YouTube will set Unlisted videos that were uploaded before 2017 to Private. You can opt out of this to prevent videos you've embedded on AO3 from becoming inaccessible.
Update to older unlisted content - YouTube Help Changes to Unlisted Videos Uploaded Before 2017 Starting July 23, 2021, older Unlisted videos will be made Private unl support.google.comAdding: if you embedded a YouTube video into an AO3 work and you have video settings as unlisted so that only people with the link can find the video, this change from youtube will reset your video to private, meaning only the uploaded (you) can view it.
You'll need to follow the instructions in the link in order to opt out of the change and keep your videos viewable to others.
Update for all fan #vidders. Starting July 23 unlisted videos that were uploaded before 2017 will be automatically set to private & current embedded links on AO3 or other sites will be disabled . If you want to keep your unlisted #fanvids accessible, you must opt out.
https://support.google.com/youtube/contact/older_unlisted_update
**This change impacts only unlisted videos uploaded before 2017** To keep those unlisted video links still working and not lose your comments or views, use the opt out process
Reason for this change (they claim) is for a security update. You have the option to re-upload a new unlisted video to take advantage of the "security" update, but you lose your comments and your views https://support.google.com/youtube/contact/older_unlisted_update
Note they say they will email you an acknowledgment once you submit the opt out ("Your response has been recorded and emailed to you for your records.) I have submitted 2 so far & have received nothing. If not, check again in 24 hrs.
Also note if you have multiple email YT channels under different emails, you need to be signed into the channel owner account to submit a proper opt out for that channel. This must be done at Google level, not Youtube.
Ex: I have two YT channels. One under MyMain@gmail.com and one under OldYouTubeAccount@hotmail.com When I surf the web I am always signed into MyMain@gmail.com as my default
To submit the opt out for the second YT channel, I had to log out of MyMain@gmail.com at https://google.com, then log in as OldYouTubeAccount@hotmail.com. I clicked on the icon in the upper right to verify I was logged in correctly at the root Google level and then followed the opt out instructions.
When you are at the opt out page and you paste your Channel ID, at the very bottom it will show the YT account owner email. This is your final check to make certain you are logged in correctly.
Step by step process here https://twitter.com/morgandawn6/status/1407746599770476546