spoofing the user agent works fine
I'm continually upset by this. If you know you rely on, for example, the fetch() api, test for the fetch api[0]. Whitelisting browser user agents just means chasing your tail for forever.By all means, politely warn a user that "your browser is not tested". It's getting to feel like a marketing driven decision, where pages just about say "our site is so powerful, we only work with the greatest browser ever, Chrome, so come back when you have it".
A local airbnb competitor currently does this.
[0] https://philipwalton.com/articles/loading-polyfills-only-whe...
This hasn't happened to me in a while, except for occasionally government or bank sites which require "IE or Chrome". In many cases, spoofing the user agent works just fine for those. I agree it's bad, though.
The one exception I've noticed is Yubikey (U2F) support for Google services. Firefox has an add-on that provides Yubikey support, but last I checked, Google blocked access to those (I believe even if you spoof the user-agent).