Berlin lost over 1.5m inhabitants in the time since 1945 to its lowest point after reunification (due to being a divided city without much industrial jobs).

At it height 1 in 5 apartments were empty in Berlin which pushed rents down below 4 EUR per square meter. A 3 bedroom apartment for less than 500 EURs a month. This was de-gentrification the parent mentioned.

Since 2010 population grew and now Berlin has housing shortages like every other capital in Europe. Rents now top 20 EUR per sqm.

Sure it's mostly about cost of living but also relatively good access to abandoned buildings (and perhaps other services) that could be used for non housing purposes. A lot of Berlin clubs and art venues started in buildings that were abandoned if I understand correctly.

I recall that there were interesting similarities after depopulation events like the black plague. Suddenly there's a surplus of built infrastructure.

The city got expensive, but then other cities in the east are still pretty affordable. Leipzig, Dresden, Jena..