I've seen sizers at the gate for DL, UA, AA, AS, and WN. I've flown WN/DL only a couple times this year, but on UA/AS have only had once in my recollection seen the GA force the sizer test. She was on a power trip as later saw much larger bags rolled on.

The reality is most marketed roll-on whether glider / spinner while they fail the sizer will fit easily into overheads, the older 757/767 the don't fit efficiently width wise but still fit.

They are there if that is the route they use to filter when the flight is full and they know they will slow boarding with the need to roll off bags that have no space. The newest overhead enable most to fit up and width in and rarely have I seen even a full plane need to check, YMMV