Per my link above from NATO, the earliest mention of a specific 2% was in '06, although the general non-specific requirement goes back to 1949 & the Washington Treaty and it's been a common definition of defense expenditure since the early 1950s. Every "agreement" or "commitment" since was just reiteration of that, because like you said,
they did the same in 2014.
HERE is a link to the '23 Summit, where again...
It's no more a "rule" in '24 than it was previously and cannot be "enforced" by NATO, as a collective treaty has no enforcement power over a soverign nation. You can only encourage/pressure other members to fulfill their commitment, which (right or wrong approach) is what I believe Trump was trying to do. There is no mechanism for enforcement that I'm aware of. Do you have any further information on what the 'enforcement' you were referring to consists of?
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HERE is further reading on the subject)