It's not just them being nice, it's the law! As an employer you legally cannot deny a request for working part-time unless you have a halfway decent reason.

In my case I was able to start working 32 hours a week at my previous employer, even explicitly communicating that I was going to work (teaching CS in high school) on my day off. I initially requested 30 hours, for working 7.5 hours on the remaining days, but they figured they should set a company wide minimum of 32 hours for people in (more-or-less) managerial positions. If I had pushed it, they probably would have budged on that too.

It is the norm to work fewer hours in many sectors, especially if you're a government worker, but in tech it hasn't been my experience (as an expat in NL). If you don't have kids it seems to be 40+ hrs or nothing and requesting anything below full time is "unfair to other colleagues" or "signals a lack of commitment".

Also, some Dutch companies and jobs within the government let you work 4x9 hours