The unjoinable surface type doesn’t work. Weld surfaces connect to unjoinable surfaces as if they were smooth surfaces.
According to the Wiki, unjoinable “makes two surfaces unable to join no matter what” and weld will “join to any surface except for Unjoinable.”
Using the most recent version of Studio, I can join a weld surface to an unjoinable surface.
It looks like this has never really worked as intended. It only prevented the creation of ManualWelds to other Smooth surfaces in Studio when Join was set to “Always” and was otherwise identical to Smooth.
We have some other changes to surface joining coming up soon that makes these joints pop up less often on accident. We’re also considering changes to the studio join tools in the future as well.
Instead of making a somewhat risky behavior change to something that has been around for so long, we’ll most likely remove the Unjoinable type entirely and convert it to a legacy enum that evaluates to Smooth instead.