DS1 got through his school lessons just before Covid disruption.
DS2's class "donated" half of their swimming time to the class between them that got the brunt of the restrictions so that they could have more than a few weeks.
The standard of swimming at their school is high, bolstered by most having private lessons. The school instructor finds they tend to have the highest proportion of swimmers out of the schools he teaches. Many of the schools in deprived areas have a handful of swimmers, and many with no experience of swimming.
DS2's post-Covid class was still better than average, but there were fewer strong swimmers than DS1's class 2 years earlier. Less time spent in lessons, private and in school. Many finished learning earlier because of the disruption. DS1's class had no non-swimmers at the end of school lessons; DS2's had a handful that never got as far as trying a length and about half couldn't strongly swim 25m.
Family sessions were really hammered by social distancing policies. While my two were lucky to continue lessons whenever they weren't prohibited, it was about 15m that they went without a casual fun swimming session. We didn't take up the option to private hire half the pool in a family bubble!
Because DS2 was lucky to have a provider that maximised lesson provision and I've kept him swimming to stage 7+ he is now one of the strongest swimmers of his class as many didn't progress beyond where they left off a couple of years ago. He hadn't expected to perform the best when swimming was part of a multi-sport event held last term.
There is a large number of children that lack opportunity to master swimming, let alone be strong swimmers and there are a multitude of reasons for that.
Covid rules removed a lot of opportunity and often the only opportunity for many for 18m to learn to swim, and that takes a way access to a fairly cheap, and very accessible sport (in terms of health/ injury)