My last set of AC flights a few weeks ago, were domestic

And there were AC personnel checking Carry On Bags both at the Check In Kiosks / Checked Bag Drop As well as at the Gate for “light packers” They were stickering approved carry ons

And they were not beyond telling some folks their bags did not meet size requirements Either by presenting them with the frame... or just factually “that bag is too big” (Gotta remember these guys do this all the time ... they know which ones pass and which ones don’t)

In some instances they were having Pax pay checked bag rates While in others, I saw them tell the person to be aware it was oversized, but could check it for FREE as the plane was full (oversold) and they were already going to call for volunteers to check their carry ons at boarding

A lot seemed to depend on the airport ... what their marching orders were for the day And how busy things were / overbooked the flights were A lot of highly stressed AC Check In & Gate Personnel IMO

Want to be 100% safe of no added fees... take a smaller bag that you know will fit

Otherwise it’s a case of taking your chances And a roulette game of pay vs no pay A fine game if you are willing to pay the worst case scenario on all flights Not if you are in a big rush... Or going to complain about the outcome even though going in you knew you were outside the parameters / rules