Germany has consistently had inflation in the single digits for the last 30 years or more.  So, a cap of 20% on rent increases likely had no effect on anything.  Since lack of housing supply is the problem, it seems counter intuitive to endorse a solution that everyone recognizes will discourage the creation of new rental housing.  I think the answer lies in tackling building and zoning codes that inhibit ultra-small ultra-low-rent housing and drive up the cost of housing generally.