Apple not supporting High Sierra?
Well here's a new one, and I'm flummoxed as to how to proceed. I called Apple about being unable to unlock my 2017 iMac (with AppleCare!) using my Series 4 Watch. Since I can't update to Mojave yet (production machine requiring some legacy software and frankly I can't risk the instability) they refused, flat out, to continue the support call.
Has anyone else seen this?
I think this is a mistake. You probably got someone who was having a bad day. I'd call back and try again. Apple is usually very responsive & helpful with new equipment + AppleCare.
I do have 2FA enabled, and have for a while. What fixed it: Un-pair and re-pair the Watch. This worked on a 2017 iMac and a late-2013 MBP. It's very likely I got someone with an axe to grind because he started gyrating wildly when I explained, up front, I wasn't updating to Mojave on a production machine that I rely upon for client work. He became stand-offish immediately.
Odd that in the hundreds of interactions I've had with Apple THIS one didn't merit a "tell us how your call went" email...