Re: I'm puzzled...
In fact, I think WhatsApp Web was my first experience of being blocked based on UA in nearly a decade
It's been a while for me too until recently, but the practice is experiencing a resurgence, it would seem. Netflix does it, lots of banks do it, Google does it with KHTML now... apparently, a return to the bad old days of "best viewed in Netscape Navigator" is in order. I'm a little annoyed that Vivaldi devs were compelled to do this... it should not be necessary, and having devs of smaller browsers give in to keep their user complaint levels down only encourages the idiots who have sniffer scripts. I know they're going to take the blame for websites blocking on useragent, but you don't overcome stupidity by giving in to it.
Also, "supported" means "we'll give you assistance (support) if you need help using our service or product." "Unsupported" does not mean "we will block you." That's "prohibited." If you mean prohibited, say so... stop hiding behind euphemisms.