[dcl.ref]/5:

A reference shall be initialized to refer to a valid object or function.

Is it an object within its lifetime? Or an object containing a valid value? In a valid state?

[basic.life] says that binding a reference to a virtual base class to a glvalue denoting an object not within its lifetime results in UB. If assume that the list of UBs from using a glvalue denoting an object outside its lifetime is complete, then the conclusion that binding the glvalue to another kind of a reference does not result in UB will follow. And we should conclude that "valid" is not about the lifetime.

Also, according to the style guidelines, "shall" shall be used to put a requirement on a well-formed program. This rule is unlikely to be a diagnosable rule.