

Are you sure? They’re both unvoiced th, which is what thorn is for if you intend to distinguish.
I can’t tell whether Old English used eth for those words early on - though the unvoiced quality in modern English makes that seem unlikely. Did we also devoice them? Eth died out fairly quickly in favour of thorn in all cases, voiced or not. Possibly because its name is “eþ” not “eð”. It doesn’t even use itself. (Though, ironically, ‘w’ also doesn’t and it replaced ƿynn, which does.)
There was another commenter - actually might have been the same guy, I’m not all that sure - who did use eth for voiced instances, to similar controversial effect in comment sections.
It’s not Linux native according to the icons in Steam, so try one of the Proton compatibility layers.
Proton Hotfix wouldn’t run it for me but Proton 9.0 got me into the game’s prologue and difficulty select (which requires walking around). Haven’t played any further than that though, so can’t tell if there are to be any problems with other levels.
Was able to modify sound, graphics and mouse settings with no trouble too.
Edit: Made it through to the first intermission screen with no problems. Well, at least not software related. Skill issues are another matter.