mainly from those who’ve sent their summer borns to school at age 4 - I sense slight defensiveness.
In my case of being against deferral you are mistaken OP. My dc started school when intake was still staggered across the year. I dislike it from my experience of working in nursery, reception and year 1.
There are undoubtedly some children that need to be in early years longer than others, but without fail, ime they are children who were premature summer (not spring) born or who have SEN. Not necessarily diagnosed SEN, but it’s often obvious to staff that a diagnosis will come at some point.
When children don’t need to be deferred but are anyway, it is detrimental to them and their classmates.
Reception classes now plan for the needs of just turned four year olds the same as they plan for all the other four year olds and the ones who turn five early in the year. Teachers adapt teaching to the children they have.
I’ve known deferred children struggle with friendships because over confidence in comparison to peers can mean that others find them dominating so don’t want to be friends. Or they have been bored by the summer term of the end of every school year because they have had more time to develop academically so need less support to get them to expected standard and the teacher has no choice but to leave them to be more independent while they help others. I know one group of parents who quite obviously distanced themselves from a family when they found out the child was a June baby who should have been in the year above. It was sad and uncomfortable to watch, because obviously it wasn’t the child’s fault.
The problem for others with having an 18 month gap between the oldest and the youngest child is that the teaching for all children has to be spread more thinly because there is a wider range of needs and a finite amount of time and resources.