I read somewhere that other people had issues with GitLab, because they had certain browser feature disabled (by enabling some tracking protection or similar).
And apparently (at that time) Gitlab uses (or used?) those features to establish that the user has what "they believe to be an actual web-browser, rather than a bot". I don't know how well that works, or how much it affects real users... (always a danger, if you try to fight trolls

SCNR).
So if some browser does not support those things...
I have (long ago, and then there were less browsers than today) be involved into running some websites. The amount of work it takes to just support even only the "popular" browsers correctly... And a great many users wont use a websites if they don't look and behave "nice". That is not just JS. Download the css from any bigger site... It's usually massive. And loaded with browser specific extensions... But, if sites don't do it, (many) users go elsewhere. So loosing "purists" is just less of a loss, sadly.
Mantis:
AFAIK No one wanted to do the maintenance work.
Forwarding bugs...
I haven't searched for where they were mentioned (haven't even checked this thread)...
But it is always problematic. Doing so, then means to have to reverse forward all and any question that developers ask on the bug. I guess not many people t want to play valet.