As I commented further down, that's true. Although category III is about buildings built before 1953/1945. I just didn't want to give people the impression that Vienna has relatively affordable rents because it has a hard cap on all rents. Newer buildings (not as old as 1953 seems to count as new) with index-bound rents have raised rents quite a bit in the past 5 years and have made Altbau very sought-after.

Fair point.

It's not only buildings built before 1953/1945 though, social housings (Wohnungen in gefördert errichteten Mietwohnungshäusern mit mehr als 2 Mietgegenständen) are capped as well.

add to that that in any new building 2/3rd of the apartments must be social housing.

This I didn't know. TIL. Also, thanks for the sources.