Do you have a source for this? I know a lot of people at Google and have never heard of this. In fact, a few years ago I explicitly asked a Google recruiter about this, and they said it wasn't an option. Is it a more recent thing perhaps?
It was absolutely an option 5 years ago when I worked there. I knew lots of engineers who worked 80% and took every Friday off.
Not sure if your manager is forced to give you that option but it didn't seem to be hard to get at least. I think by default such requests gets accepted and your manager would need to give a good reason why you couldn't work part time.
I think it depends on the team and your role in it. I worked at one of the Google X moonshots and never heard of this. There were a couple engineers from other teams that were donating one day a week to helping us, which is a similar 80/20 split. These 20% were somewhat common at Google, I believe, but it was never to take time off work.
