As determined by whom, with what criteria and what methodology?
> or how ludicrous the reason for the breakage is
This behavior in MooTools (and Prototype.js and probably others) of monkey-patching built-ins was widely considered ludicrous at the same time as they were widely used across huge chunks of the web.
A more recent example: I consider real world use of alert to be ludicrous for nearly every usage I can imagine, and I hate when I interact with its usage even for benign things, but when Google tried to remove it from Chrome I objected along with a lot of others who have no personal love for alert itself. I opposed its removal because alert has certain guarantees, and removing it would break an untold number of sites in totally unpredictable ways. There are a bunch of “why don’t you just…?”-type suggestions in other comments, but the fact is you can’t “just” substitute anything for alert without recreating all the things people hate about it.
> I’d like to hear more details about how these decisions are actually made.
They discuss some of that in the article, but if you want more detail there’s a ton of public record in the various TC39 repos on GitHub[1], including repos detailing their process as well as per-proposal repos demonstrating application of their process. If you’re particularly interested, you’re in for a lot of good reading.