Have you tried going to church and making babies?

Serious question.

x87 has been functionally superseded (largely by various SSE iterations), but it is still supported for backwards-compatibility.

Modern applications and compilers largely do not use x87, but they can, and x87 results depending on hidden bits persists in modern x86_64 CPUs.

DLang allow to use it explicitly by using Real type, I instead of Double and Float types.

yet it is still used for arithmetic with "long double"

Wouldn't that resovle the issue? long double contains the whole 80 bit state, not much space for undefined bits left.

Yes, it resolves the issue in practice. However, the issue is an implementation detail of the compiler, not of the floating point format itself.