I thought this would end up happening, the developers of Cobalt which is published by Mojang and even shares one of the Minecraft developers has dropped the planned Linux version.
Annoying, as I purchased a copy when they mentioned Linux all over their website originally, and after waiting a long time it has now been dropped. I purchased a copy years ago before I set myself the rule of never pre-ordering, and I kept trying to keep up with their progress.
Originally Linux was due in their Beta phase (you can see here thanks to the Wayback Machine).
I spoke to the developers on reddit, who confirmed a refund for me shouldn't be an issue, but after poking them a few times it seems it's currently falling on deaf ears.
This serves as yet another reminder: Don't ever pre-order, and don't ever buy on a promise. Buy a game when the Linux version is out, never before.