I have an extension (mostly for my own amusement) that embeds a youtube-nocookie embed in an extension page. This has worked fine until recently, where recently is somewhere in the past ~7 months. From what I understand the problem is that there’s no way to send a Referrer or Origin header when the embed is loaded and youtube thus doesn’t serve an embed anymore.
Since the extension is used mainly by me I could probably move most of that page to be hosted somewhere and it’d be fine, just not as cool as it being self-contained.
EDIT: I guess this is an unsolved problem There is no HTTP Referer header for iframe in an extension page · Issue #896 · w3c/webextensions · GitHub