The thread headline is inaccurate.

UA used to be and sort of still is 45 linear inches with no one dimension greater than 22 inches, but now it also has changed some things that specify the individuals limits for each of the three separate dimensions. There has been change between the start of 2013 and today in some of its binding representations to customers on this matter.

AA used to be and still sort of is 45 linear inches with no mention of any individual dimension limits beyond that, but now it also has changed some things that specify the individual limits for each of the three separate dimensions. There has been change between the start of 2013 and today in some of its binding representations to customers on this matter, but it depends on what you may be comparing or not comparing.

DL used to be 45 linear inches with no additional mention of the individual dimensions, at least in some of its binding representations to customers on this matter.

By the way, the contracts/conditions of carriage, tariff filings, the airline websites and the government-filed cabin baggage policies are not all one and the same. That is the problem with relying upon airline websites to try to figure out what has and has not changed and when, because the big 3 US cartel kingpin airlines have not even been internally consistent beyond the combined 45 linear inches element.

Welcome to the results of these airlines being individually inconsistent even within just themselves.