I don't quite get this concern about proprietary JS. I agree JS can be malicious, but it has nothing to do with license. Or you can say you don't want to give your data to Slack.
I sympathize with those who object to proprietary JS, but in my case I'm mostly annoyed at having to run code just to read static content - quite heavy code, too. When I have a lot of discourse.julialang.org pages open, my browser noticeably chugs.
Often it has nothing to do with maliciousness or data aggregation, it's a matter of principle: not supporting proprietary software.