Every so often I've seen them in crackdown mode where they send people to the sizer. There's usually an extra agent or two at the gate when they're doing that. A lot of people try to bring things that are too bulky and would not fit. Maybe 95% of the time they're not cracking down but it's better not to push it (and it really needs to fit in the bin on the plane).

Much more enforced is having to pay for the carry-on if you haven't already - if you're in zone 2+ and take something that looks like a carry-on you have a high chance of getting stopped. Have also never seen them enforce the weight part, though it helps to be muscular yourself and able to handle it.

Have also seen lots of people trying to stretch the personal-item definition. On the carry-on my go-to is a Travelpro or other rolling bag sized to the specs and probably bought at a thrift store. On the personal item it's a tote bag that zips at the top and can be stuffed if needed but is also flexible so that if put in a sizer, it'll smoosh (obviously don't put fragile items there). Things like coats and hats can go on your person, as already mentioned, even if it's July but you're carrying back a winter coat u bought on the trip. People also can dress in multiple layers just to cut down on clothes volume in the bags and make room for other things.

With more people doing these discounted bundles or having carry-ons included in some sales, the bin situation can start to resemble the legacies. When that happens the FAs get insistent about only putting carry-ons in the bin, not personal items.