The Mietendeckel has effectively triggered the death spiral of Berlin's housing market, it's the product of a reactionary local politics and the intransigence of the main political parties at a federal level to either fund housing development or loosen planning restrictions. I came to berlin in 2011 and almost a decade later the problem has only gotten worse, which goes to show that the direction of cracking down on private landlords whilst making negligible efforts to either zone new housing areas or speed up planning permission is ineffective at creating more living space for the 140,000 people who are under-housed.
The death spiral is accelerated by two factors, firstly private landlords have had their income curtailed and are selling up, secondly prospective private investors in rental housing, wary of further unilateral retrospective changes in the future, are walking away. For those that don't know what the Mietendeckel is, rents in Berlin have been reset to circa 2013 levels, where buildings built before 2014 cannot be rented above 6-10 euros per sqm, existing contracts and renovation investments be damned. For example a building in Friedrichshain built in 1890 but completely renovated < 10 years ago can only fetch 6-8 Euros per sqm, if an elevator is present, underfloor heating and a high quality finish to living areas is provided you could at a maximum charge an extra 1 Euro per sqm. Failure to retroactively lower the rent for your existing tenants is punishable with fines up-to 250,000 Euros, there are practically no loopholes.
So this legislation has lead to the current situation where landlords are preferring to sell their properties when they become empty or when that's not foreseeable, selling them with the tenants in them (which can reduce the final sale price by up to ~30%) or just hold off on renting empty properties for now or even rent them on the booming black market. This is evident in the 47% drop in rental listings for apartments in old buildings and the moderate rise in condominium sales.
German tenancy laws read like the wish list of rental activists in America or the UK, they provide comprehensive protection from no fault terminations and are backed by a lethargic court system, the sole reason that can be given for terminating a contract is when the apartment is needed for you or your relatives own use, a lot of people like to imagine that this is abused but the consequences are so severe I can't see any logical person making that choice. A long term tenant who fights this kind of termination can expect to stay in their apartment for up-to 2-3 years, but in the end will face certain eviction and ruinous costs in excess of 15-40,000 euros. Whilst there are clauses to protect vulnerable tenants from eviction, they are not evident in practice, just recently a 82 year old woman with advanced dementia was evicted as the court determined after 3 years that she could be physically lifted out the apartment by her daughters and so is therefore capable of moving elsewhere.
There is nothing to rent here, your sole choice is to buy a place, but you won't be able to afford an apartment which is unoccupied as the banks' valuations don't match that of the realtors. Which leaves you with finding soft targets for eviction, the easy to persuade will have accepted an offer, so take a pick between the elderly or single earner families with kids.