If I'm reading it correctly, the paper says that unreachable is not allowed in a constant expression (because calling it is UB).  That is a breaking change for calling assert in a constexpr context [assertions.assert].  And even if we fix the one in the standard, how can users write their own?

Don't get me wrong; I'd love to see this become a required warning/error if we can do it.  But the devil is always in the details.