My area had primary as yR-y3, and y4-y7. This became an issue when the National Curriculum and Key Stages were introduced and we standardised in line with the majority of the country. In y5, we were told that we would leave at the end of y6 and form a double intake with y7.
Our secondary school had a couple of years notice to recruit staff and expand with a mass of temporary huts to accomodate an additional year group (and also a growing roll from 6 form intake through 7 to 8)
As it happened I moved areas, was 6 weeks away frim completing infant school in y2 and was mentally ready for juniors then found myself in y3 still in juniors. Then never did a final year of juniors as the usual y7 opportunities like the residential or prefect places weren't opened up to us. I remember some years later, my beloved form tutor saying that prior to us starting, not only were we a year younger, we also didn't have some of those developmental experiences, and different schools prepared us differently. Our head teacher decided to teach us French a year earlier to prepare us, but that was largely repeated in y7 because not all schools had.
Repeating a year is logistically unfeasible down the chain (staffing, accommodation)
It's also emotionally and developmentally impractical. Pupils have been mucked about with enough. Also, it's not one neat school year, it's dragged through two year groups with some recovery time in the middle. While I worry about my children who loathe home learning (one with SENs) it's not helpful to hold back and under-stimulate those who carried on progressing.
Adjusting more superflous curriculum requirements, and funding and resourcing support is the more practical answer.
Maybe, particularly for GCSE/ A-Level progression, summer schools could be set up- not extending the term, but externally based with teaching/ college staff opting in to run units with students booking in to specific areas where they have omissions.