It is doable and in some countries like Switzerland even a common practice to offer an 80% - 100% workload (if you look at https://swissdevjobs.ch/ or any other tech job board you will notice it occurs quite often)

The problem is that an 80% workload for companies also means 80% of the salary which makes sense if someone wants to work part-time, but it's not the implementation of a 4-day work week idea.

Personally, I believe a 4-day work week will sooner or later be a standard, simply because Developers (including myself) waste a lot of their daily time and by having a constrained time-frame they will also be more efficient. In the same time (hopefully) management might reduce the number of meetings forced onto the Dev team.