The job market is mostly binary in the sense that people are either employed and work 40h/week or are unemployed and work 0h/week. I can't figure out an economic explanation for this. My intuition tells me that for the market to be optimal and more efficient it would also have to include the percentage of how much you work.
I also believe that it's easier to extract more value per hour from a programmer that works 20h/week than a programmer that works 40h/week, so why not adjust for that?
Am I missing something?
However, in all of the times I tried to do this part time work, I failed to strike the balance I wanted. The reason for this is that your colleagues aren't working the same schedule is you. They may not know your reduced schedule (even if it's in you calendar) or they may just not care. But it's very hard (at least for me, YMMV) to ignore phone calls, texts, slacks and emails when your teammates (or people who report to you) are asking for help with something.
So I think for this to work, it would need to be ingrained into the company culture.