As I said, there are also a number of watch faces that will show altitude.
Try myABC again on the 645. There was a general slowdown with CIQ on some devices a few months back, but that has been corrected now.
The 645 and va3/va3m all have baro altimeters but none have an altitude widget, so it's not unique to the 645.
The watch is advertised as having a barometric altimeter and surely it's false advertising if you have to go to the effort of starting an activity just to quickly check your elevation.
But there's no technical reason why the "running" watches can't do this as the hardware is already there.
You're correct. There is no technical reason. It's about marketing and money for Garmin. Feature omissions on devices has been discussed and explained in countless threads throughout these forums. It's Garmin's choice what features go into their devices regardless of what we as consumers feel belong there. As consumers, we can use Garmin's product request site and request features, or not buy their products and go with another manufacturer.
I think we all know that all of these new watches have the same internals, albeit different battery sizes and face designs, and are marketed differently to target different consumers in order to maximise sales.
Does the 645 have a barometric altimeter or not? If it does, it's not false advertising because you can't utilize it the way you want.