Being a contractor in a niche technology (in my case it's Perl) is great for this, because the companies involved can't really say "work more or we'll find someone else" – we both know that they won't ;) Also, beyond 4-5 hours a day my (and afaik everyone else's) productivity is diminishing anyway, so it's not like they're losing much because of it.
The company is imo a great place to work, but this is also in Norway where it's pretty common for people (usually older ones, though) to work 80 or 60%. That probably made this a lot more likely to happen.
I'll add that there was some fear that everyone else would want the same arrangement and that we'd suddenly only have devs working part time, but nobody else has expressed any interest in this. Different strokes for different folks...
Dunno if you're interested in the whole process, but I documented it on a blog post in case someone is after the same solution (https://givemethechills.com/why-did-i-start-working-part-tim... )
BTW I first got the idea right here on HN and the good thing is that I think, as developers, we have one of the best jobs that allows reducing our working time.
To improve it even further I'd love to do this kind of work entirely remote, thus givin me some extra time to spend with my family and removing the commute. That'd be the max :) I can't ask for this now because we're not a remote company and I know for sure that if the company is not fully remote I'm gonna fail badly (mostly because I'll be the only one doing it ).
In any case, after doing it for many years I can say that it's amazing and I love it