I found half an answer here 
@david , It’s a shame and if I understand it properly it’s absurd.
I own an iPhone 12 mini under iOS 14.8.
Because of your policy, and obvious love for planned obsolescence, I can’t login on any Discourse community anymore, and users are let alone with 2 choices.
Either they upgrade their phone, which will make the phone slower, as always, because manufacturers want that,to push customers to buy new devices every 9 months;
Or, they stop using Discourse because they cant login. Because Discourse put an unnecessary piece of js code somewhere, to block “old” browsers
I seriously doubt that a login button needs the latest version of the latest software.
This is totally absurd.
I won’t buy a new phone because of Discourse,
and I won’t upgrade because of Discourse neither.
Ciao Discourse 