As a process of elimination, this is not being reported by many others, mods are involved in the initial reporting of the issue, therefore a process of elimination shouldbe followed, starting with trying to play on the PC without Mods first, to see if it works. And then, if the left-click is not working, slowly add mod by mod to see if/when the issue starts.

I would also check if the mouse is a game-mouse, whether there are driver updates, software updates as well. Unplugging and replugging the mouse may even help.

My left- (and right) click work and I have mods. The left-click works Solo and online. Unless others have the same issue, or Funcom can replicate the issue, it will be on @Spiegel to comment whether it has been resolved, or whether there are any further issues.

Checking the informal Conan Exiles Discord might be a good idea in checking if others have ideas/solution/same issue.