The game’s minimum system requirements on Steam include
Network: Broadband Internet connection
It’s a different matter to argue that Single-Player shouldn’t require constant connection (that’s something I can agree with), but you can’t claim that you didn’t get what you paid for if that item is listed on the system requirements. It’s not something unique to Conan Exiles, and it’s not unique to games sold on Steam. In fact, it’s irritatingly common these days.
If you don’t meet the minimum system requirements, and still choose to buy the game, hoping it works, it’s your choice. If your gameplay is dependent on a mod, it should come as no surprise that it could cease to work at any moment.
I use multiple mods in my game. If an essential one breaks, I will basically lose over a thousand hours of gameplay. That’s the risk I take when using mods, but if that happens, the responsibility for that loss is mine, and mine alone. Not Funcom’s, not the mod author’s, but mine. I’m the one who chose to use the mod, with full knowledge that it might go unsupported at a later date.