That's becoming more and more widespread. It's idiotic and counterproductive but apparently companies love some fake sense of security.
I think this is more for "traditional" companies, I've never seen a startup here (Berlin) do that in the 10 years I've been living here.
I have 3 months notice. It is unreasonable.
It's unreasonable for the company to want you to stick around for 3 months before you switch jobs, but it's not unreasonable to ask a company to let an employee who has been with them for years 3 months' (notice or severance) before they fire them (as the power balance is not equal - the employee is supposedly dependent on that salary for their livelihood and not everyone can instantly land a new job after being laid off).
Overall even as a business owner myself I prefer the pro-employee German system over the free-for-all American one.
Here in Germany, in fact things are problematic in the other direction. For the last 2 hires I made, their previous company forced them to work through their 3 month notice period.