I know they do it when the battery gets really bad, they could move it up to happen sooner. I just imagine it would fit into the planned obsolescence narrative too easily.

My 2014 MBA has recently started saying this, but I haven’t noticed any sudden change in battery life, I still get 5-6 hours.

It’s based on a certain number of charge “cycles” and not necessarily actual remaining capacity. When I was working as a “Genius” equivalent for an Apple reseller about five years back the number of cycles before that warning would pop up was around 500.

500 seems like a fairly low number to me, I'm surprised it's not more like 1000.