[lucideer]

Youtube didn't change - browsers did. Embed loads plugin content: browsers broadly moved away from plugins (they'll still technically work if you feel like jumping through installation hoops as a user but they're not maintained and not broadly preinstalled as they used to be).

During this broad migration by the browser community away from supporting plugins within embed tags, there was likely some audits of how widely they were relied upon across the web, and compat hacks put in place to account for the most common uses.

It's very likely Youtube removed endpoints after the browser hacks were in place for some time.