They reached that point because: a) everybody agreed that they were a good feature, with precedent in other languages; b) the spec text had been scrutinized by the people who like to spend time scrutinizing spec text; and c) two implementations (Moddable and V8) had successfully implemented them. That's the process, and it was followed to the letter.
Does that put a burden on other engines to keep up? Sure. But that's what we signed up for. The webcompat bug you linked is from April 2020, more than two years after lookbehind assertions were standardized. That's nobody's fault but ours, and we did this project so that we don't end up in a similar situation in the future.
I'm not sure what your source is for JS regexps being impossible to implement from the spec, but if you have specific points of concern, you should open an issue [6]. Writing a spec is hard work, and things get missed, but the people working on this are genuinely trying their best.
(As far as I can tell, Canonicalize is what everybody thought they were implementing. V8 tried to get clever with ICU and ran facefirst into some of the dark corners of Unicode.)
1: https://github.com/tc39/notes/tree/master/meetings
2: https://github.com/tc39/notes/blob/master/meetings/2016-11/n...
3: https://github.com/tc39/notes/blob/master/meetings/2017-03/m...
4: https://github.com/tc39/notes/blob/master/meetings/2017-05/m...
5: https://github.com/tc39/notes/blob/master/meetings/2018-01/j...