At the end of the day, it comes down to whether browser vendors are willing to ship the feature even though it breaks said websites, and they do have visitor data to back those decisions. If the rule was "never break any website ever", you'd be able to block proposals you don't personally like by crafting a website that "gets broken" by that proposal.

At least in the array grouping proposal, they did evaluate amount and popularity of possibly breaking sites, before eventually renaming the method from .groupBy to .group

https://github.com/tc39/proposal-array-grouping/issues/37