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.