after switching to a browser-like user-agent, YouTube started responding with a "supported browsers" message rather than the actual video meaning bookmark creation failed
when trying other user-agents it was discovered that nytimes.com (why the user-agent was originally changed) had a problem with the github.com address in the user-agent. By switching to using https://ghost.org instead the request was allowed through fixing both YouTube and NYTimes embeds
https://github.com/TryGhost/Ghost/issues/20445