... Hence my previous comment you quoted :
... and I was strictly referring to "Moonchild Productions", are you officially part of that team now?
Joking here , of course, but my comment was made on Feb 22nd, your fix landed on the official UXP repo on Mar 10th (and has yet to land on a roytam1-compiled binary), so yes, I wouldn't have survived had I held my breath...
Jokes aside, my contribution was just to identify the missing JS feature; the report was made here by a UXP-fork user (on an unsupported OS), so the proper way to tackle that would've been:
1. Get access to a machine with an official UXP sanctioned OS
2. Get hold of official UXP-based browser binary/binaries
3. Reproduce the "bug" (rather the platform's deficiency) there
4. Register an account in the official PM forum
5. Report the problem in their WebCompat subforum
(few fork users on XP/Vista can/are willing to go through the above ...)
... and then hope one member of the under-staffed MCP team takes it upon themselves to attempt a "fix"; given that the problem, as reported, affected a Polish government web site (relevant probably only to Polish citizens), MCP staff would have little incentive to spend time on fixing that, when other major/more popular sites are broken... So yes, over the years, I've become somewhat pessimistic ...
Thanks again for implementing "object.GroupBy" .
Regards.