I’ve got an Apple Watch Series 6 that has had an always on clock face for 5 years. In the flashlight mode in the lowest brightness I can see burn in, but nowhere else can I see any sign of burnin.

So oled burn in is not a totally solved issue but it’s not a big enough deal for me to care when the literal worst case scenario of an always on clock face does not cause real issues after years.

It would be amusing if they enabled autohide by default, for those models.