I've heard (but never checked, although I did find some resources: https://www.hensche.de/Rechtsanwalt_Arbeitsrecht_Gesetze_TzB...) it is mandated by law that employer (as long as it has more than 15 employees) must allow you that.
The 20% paycut is pre-tax, so the real cut is a lot lower. It also lets employers hire a 5th person for every 4 80%s, so total productivity is likely higher due to people being happier, and there's more jobs total.
It's hard for me to imagine why tech companies wouldn't want to offer this to all their employees.
“Equal employment conditions
You must offer your part‑time staff the same employment conditions as full-time workers, including pay, leave, notice periods and other rights and benefits linked to their employment.
Modifying working arrangements
Whenever possible, you should try to accommodate requests from your employees if they want to change their working schedules, such as:
- transferring from full-time to part-time
- transferring from part-time to full-time
- increasing their working hours
You cannot dismiss an employee if they refuse to transfer from part-time to full-time work or vice versa.”
Working hours per week by country in Europe (includes some non-EU countries) at https://ec.europa.eu/eurostat/statistics-explained/index.php...
I'm sure there are other similar cases, but overall it's pretty rare here.
In the EU, employers have to, by law, consider reasonable requests for flexible working. If you work for a large company, that should mean most will have the option.