It's not mandatory since 1985, and apparently the number of schools offering it has declined quite a bit to about a third in 2016[1], and I wouldn't be surprised if it was even lower today but I can't find more recent figures.
It's a shame, because it's both useful – potentially life-saving useful – and fun. When I lived in Asia I tried teaching some adults to swim, and even just floating on water is hard for most.
It's not mandatory but as far as I know it's pretty much a given that you send your kids to swimming lessons. People that can't pay for swimming lessons for their kids can request government help to pay. Generally when kids are about five years old they'll start with "Zwemdiploma A" ("swimming diploma A"), which you often need to even be allowed to enter the deeper pools.
When you don't send your kids to swimming lessons you potentially deprive them of fun days away with their classmates because they need that swimming diploma to be allowed to come.
It's also a lot easier to teach kids than adults. Besides logistical things like the ease of pulling a kid around versus an adult, by adulthood a lot of people end up being embarrassed around trying new things, especially ones that require them to be largely undressed and in a strange environment.
As a teenager I ended up carving our a niche among my fellow lifeguard/ swim teachers for myself teaching adult men to swim. I suspect it was their pride stopping them from taking lessons from the (far more prolific) girls, who often times were teaching their kids to swim.
And not just that- 10% of Dutch traffic deaths are by drowning.
I did notice when I lived there that people found swimming lessons very important, to the extent that cards to congratulate children on passing their swimming test are commonly sold.
