As disclaimed, it turns out there were holes in my understanding. :)
The Ars article being discussed is actually where I read that. However, upon attempting to find the original case it looks like SCOTUS hasn't yet heard such. From this Lawfare post, (https://www.lawfareblog.com/fifth-amendment-decryption-and-b...), SCOTUS Fisher v. United States is relevant but doesn't actually involve passwords. Also from that page, the Eleventh Circuit seems to think that government knowledge of incriminating device contents permits forced decryption while the Third Circuit argues that merely knowing you possess the password is sufficient.
In both cases the precedent is that you can be forced to turn over a password; it's only the details of the prerequisite government knowledge that are under debate. As far as I can tell, the current PA case goes against that.
Yes - there has been no SCOTUS decision on this. The current circuit split between 3d and 11th circuit will probably put a case in front of SCOTUS eventually.
The EFF seems hopeful (https://www.eff.org/deeplinks/2019/11/victory-pennsylvania-s...) that an even larger split might arise due to this PA case in addition to a couple pending elsewhere. IMO ideally passwords should receive complete and unconditional fifth amendment protection.
I agree that passwords are testimonial in nature and should be fully protected, but I have no idea which way SCOTUS will go. There are a lot of things where one can with some reliability predict the general outcome of a case at that level due to previous rulings and precedent -- this one I just don't know that anyone has a good idea which way they'll go.
