I'm in the business of renting out flats. Not in Munich. Sadly, I'm in a lower-demand market, and I wouldn't rush to classify this as "discrimination".

It is very very difficult to deal with bad tenants. They exist, a lot of them. And it is very very difficult to know who is a bad tenant through these websites and Facebook groups. The problem here is an information problem. If these sites gave the person applying for a flat the chance to give information to prove they are good enough, that would be a blessing, both for (good) renters and for landlords.

I am not talking about financial information. I would be pretty happy with some references that would indicate the person is honest and serious and easy to deal with, even if they don't have much money; some access to Facebook profile and posts or other data available at the internet would do the job for me.