Reading the
SMF forum, they are saying it is due to a browser update, which disables flash. It might be different browsers update at different times, so thats why some are still working and others don't. As it's browser policy, and also as Youtube have changed to HTML5 for videos, I doubt it'll be coming back.
SMF are saying they have no plans of adding video embed support, but there is a mod that can be installed to do that. This
one which I think should be right would need a SMF update to 2.0.11.
I do use the flash embed quite a bit, so would be happy if something similar could somehow be reinstalled, but I understand its a lot of work and increases the page load time, so might not be wanted.
Up to Claire and the mods on here