Turn-based RPGs I can understand, but “RTS” is “real-time strategy” – it’s intrinsically not turn-based.

You can get turn-based strategy games, but they aren’t RTSes.

It depends on what you’re looking for. There are more hard-warsim oriented games at Matrix Games, though a number of those are also available on Steam these days. Steam has a “Turn-based Strategy” tag:

store.steampowered.com/search/?sort_by=Reviews_DE…