shrug To do my actual job, I have to implement on deprecated APIs all the time. Deprecation is a declaration of intent, but intent rams into reality and changes continuously.

There are two types of deprecated APIs: ones nobody uses and ones everybody uses. Sounds like Mozilla mis-guessed on which one of those Shadow Dom v0 would be.

(From my limited experience: I bet while the Chrome team was trying to deprecate shadow dom v0, YouTube, which still operates as a pretty independent arm inside the Google ecosystem, built a new system on Polymer and in terms of Google's management architecture, nobody has authority to tell them not to do that. Far from an intentional shafting of other browsers, it was likely a failure to coordinate internally coupled with total apathy regarding what that meant for other browsers, since from YouTube's point of view, Chrome is free and available to everyone.

Never attribute to malice that which can be explained by disorganized-bag-of-cats management style).