Mrz I think OP isn't referring to reception. OP is talking about the NC and 5 year olds (her DC won't turn 5 in reception). OP talks about 'freer Reception play' existing until he is four (inclusive, presumably), so up to and inclusive of reception.
Reception at just-turned-four is tough. So say the stats. Only roughly half of summer born children achieve 'good' learning outcomes in reception, and little more than half of all boys, leading me to estimate that only about a third of summer-born boys achieve good learning outcomes - and that's including April born boys who are 5 months older than August born DC. Also summer born children, particularly boys, are more likely to be diagnosed with SEN.
So are summer-born boys just 'not up to scratch', are they indeed more likely to suffer from SEN? Or are the expectations for them wrong? If only about a third can 'pass' and meet the expectations?
IMO the expectations are set wrong. No, you don't have to be a genius, terribly advanced to meet the EYFS goals, even as a summer born boy. But being 'average' won't be enough. You DO have to be more advanced than the average.
Nevertheless, learning through play as it is done in any decent reception class is probably age/developmental stage appropriate for 4 year olds (just as it is for their 5 year old class mates). Just the expectations for outcomes are a bit wonky. But as no-one will look at their EYFS outcomes when considering a job/uni application, you could argue that it doesn't really matter very much if they achieve a 'good learning outcome' at this stage.
The REAL problem (and what OP presumably is referring to) is starting Y1 at just-turned-five. At finishing Y1 at still-five.
Yes, my just-turned-five year old boy was taught how to spell the days of the week. And no, it wasn't learning through play. It involved sitting still, sitting at tables, for the largest parts of his school days. And no, I do not think this is developmentally appropriate, just as it wouldn't have been for his classmates who are a year older than him, when they were in reception. And yes, having him spend the majority of his waking hours at school, doing desk-based work, at age 5, did cheat him out of the opportunity for age-appropriate activities i.e. playing, running, climbing, ... these should have been the largest majority of his days, rather than confined to short breaks and the limited after-school time.
Now as it happens, my August born boy (in Y2 now) hasn't had a problem with this. Academically he is doing fine, better than fine in fact. He can sit still and he has learned to spell the days of the week, with no tears or struggles involved. But he hates school and I can't blame him. The only thing he likes about school is break time. And I'm convinced he would have learned more, academically, if he had been at home when he was four, and if he would have had the opportunity to learn in a play-based way when he was five. (As most of his classmates did)
My daughter has an April birthday. If these new rules about leaving parents the choice about deferring reception entry for a year for summer borns come into effect in time, I will be 'holding her back' (unless her development speeds up massively in the next years), because as you say OP, anything else would be a waste of the teacher's time and cheating her of developmentally appropriate activities.