While the Underworld Ascendant team didn't manage to hit their previous release estimates for the Linux version, it looks like it's finally there.
In the latest weekly update to address the state of all things, one of the team said this:
Still aiming to release these all at once either later this week OR early next week. We addressed as many Mac and Linux bugs as we could as a result of our playtest sessions; thank you to those of you who volunteered your time to help us with this. The only blocker now is that the PC build still needs to have some fixes verified by QA, such as the thrown item bug which previously disabled items such as the Jewel of Befuddlement from being activated when thrown.
Considering it was funded on Kickstarter back in 2015, with a Steam release for Windows back in 2018 we've been left waiting on this for quote some time now. Thankfully, they did pull in a porting studio to handle it, so the end result should be a Linux version that's tested working well—hopefully anyway.