If e.g. a TL8 planet with a TL8 planetary arcology reverts to TL7 while part of a player empire, the structure will remain intact (ruining only triggers on independent worlds) but the planet's maximum population will drop to the default max for the planet at TL 7 with no hab structure.

IMO, since the planetary arcology requires a TL7 ocean arcology to build, the planetary arcology should be able to either regress back to an ocean arcology, be ruined, or continue to support the ocean arcology's max population, rather than abruptly supporting no excess population.