1. The number of websites using the old version of Mootools is vanishingly remote, relative to the number of existing websites not using it, and the number of as-yet-unmade websites who will also not use it (but who will be harmed by the committee's dedication to retarding the development of the language).
2. Of those websites using this old version of Mootools, the number that use flatten, and do so in a way that would definitely be broken by the introduction of standardised "flatten", and wouldn't be fixed thereafter is, statistically speaking, zero.
3. Most of the same people who are affecting concern about backwards compatibility in the case of the web work for tech giants who regularly retire products and APIs that are relied upon by thousands of people, and make backwards incompatible changes to their operating systems that break masses of unmaintained apps.
But the fact is, you can't make arguments like this to groups like the TC39 and expect to even get a hearing. Despite their occasional spiel about seeking outsider feedback they are, like any institution, happy in their long-settled groupthink positions, and reflexively frightened and dismissive of any challenge or criticism by outsiders.
You saw this when the whole "smoosh" affair first broke. There was a long, passionate, but civilised debate about it on GitHub (https://github.com/tc39/proposal-flatMap/pull/56 ) which was abruptly cut off by a TC39 committee member, who locked out non-contributors entirely, with the dishonest claim that things had become "too heated". What he really couldn't stand was the sight of the committee's dogma being challenged.
Remember that next time the representative of a standards body whines about lack of participation in the process by regular web developers. It's a lie. These groups are only interested in the opinion of outsiders if they align with a set of fixed and unchallengeable precepts, and they will revert to depressingly familiar suppression of speech the moment it looks like things aren't going their way.