@Scott_Barrett1 and others in this thread:
Shuji at Zwift HQ here. We’ve been phasing in Metal support across Apple platforms for many months now.
We will be requiring Apple Metal support from here forwards, and thus only the hardware Apple has written Metal support for will be able to run Zwift soon. This is the reason we require macOS 10.14.4 or later.
While 10.15 and later may unofficially run on older computers it seems that it doesn’t include Apple Metal API functionality. Generally speaking only devices from the past 9 or 10 years appear to have received Metal support from Apple.