Currently in Early Access, War Selection from Glyph Worlds is a somewhat promising looking real-time strategy game and they just released it for Linux officially. It's free to play too, so at least in terms of cost there's no barrier to getting involved and giving it a try. Looking over the monetization model, you have to pay for access to specific nations and the single-player content.
A few different game modes are included like a free for all, a survival mode to face the hordes, ranked 1v1 matches, team games and more. For a proper RTS, it sounds like it has all the basics there to build upon to a full release.
Seems most players have been enjoying it too, as it regularly sees a few hundred online at any given time. Not only that, it's being rated by users on Steam positively too so they must be doing something right at least. Like some of the classic strategy games, you rise through different ages from throwing rocks to firing tanks and more.
Personally testing it, I found everything did work as expected with the new Linux build but the game assumes you will already know your way around it. There didn't seem to be anything to help ease in new players, which I hope they work on as it closes in on leaving Early Access. At least with the Sandbox mode you can play around building and check out the mechanics first before diving into an online match.
You can play War Selection on Steam.