It was actually agreed in 2014 in conjunction with the leagues, giving the clubs five years to comply. If the clubs also read the rules of the league they play in, they would have known that non-compliance would result in relegation.
Whether 18 square metres is a bit on the large side, I don't know. However 12 square metres for 16 players, officiials, showers etc. is a bit on the tight side!
Compared to the regulations that have to be met regarding how many seats, the number of spectators that are under cover, the number of media seats available etc., this is one of the more reasonable criteria!
Personally, I think the ground grading standards are far higher than is required & far too expensive to achieve. However, many clubs pay out far more than these amounts in wages & then complain when their ground fails the grading.