Also, Custom Elements is in the area of W3C, not TC39, I believe. There could be different rules and people involved. Although I'm sure they'd like to avoid breaking the web too.
Indeed, and that's the point I failed to make. TC39 is rightfully concerned about not breaking the web. They are very mindful even if features that have reached stage 3 (because by this time everyone is already using them).
Browser implementors though... They take a much more callous approach.
And used by the biggest websites and projects on the Web (Google's own YouTube and AMP until very recently).
How many more websites are using it because it was out, and implemented?