Of course MooTools' usage in new projects is negligible nowadays, but it wasn't in the past. And there are quite some old sites that still use it.
MooTools had this terrible practice to extend native prototypes. I'm still surprised that it actually gained some favor, but the worst part is that it's been part of Joomla at the time, so it ended up in sites whose maintainers didn't always get to make an educated choice (like jQuery).
In the end, yes, there's a non-negligible amount of sites that would break. Should we let those sites break? Keep in mind that it's not always about abandoned websites, but maybe also sites of small companies or commercial activities that don't have the resources to update their sites.
But alas, we don't have data to back up that.
I still think that letting the TC39 settle for includes instead of contains (because of MooTools, again!) was a slippery slope, and this new issue proved me right.
Now, we do have alternative decent names (like flat or squash), as smoosh wasn't really a serious proposal (very inappropriate from Ficarra, anyway). But I think the adage "We shouldn't break any site when introducing new features" is just an impossible task for TC39. It's reasonable to think that every new feature has a non-zero impact on some existing website/webapp.
I'd like to see real-world data, though.
The problem is not with the TC39 per se. TC39 only reflects the consensus of major vendor; what the point of a standard that will never be implemented; e.g. Array.prototype.values which was turned off in chrome and firefox because it breaks some outdated CMS.
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