My ex-wife went to a 4-day workweek (with longer hours per day) for a few years, when our kids were smaller. A friend of mine did the same for a bit (albeit, to be fair, in a pretty small company).

I'm sure there are plenty of people who were refused the chance (some companies can be atrociously dickensian even today...); but on average, flexible arrangements are less uncommon than you might think.