OK so, I tried forcing Proton "compatibility mode" for Wurm Online in Linux, using 5.13-5... same basic result. But I installed Proton-GE (GloriousEggroll, google for it) and Wurm Online runs! It's very slow until assets load... like, less than 1FPS. You have to watch it stutter for 5 minutes. Then it starts working smoothly, but performance is definitely not "OK". I might try to take this to some Proton/GE forums and see if we can't get a workaround here.
I have a feeling there is a glibc6 incompatibility between the existing Wurm Online distribution files (Steam version AND the standalone from Wurm's website) that will require a recompile on the company's part. This isn't something that can be fixed by the end-user if you are using an Ubuntu 20.04-based Linux distribution.