Does const reference have external linkage in C++?

The reference itself isn't const, just the object it refers to; so (arguably) this rule doesn't give the reference internal linkage.

It doesn't make sense for a reference to be declared const. The C++11 standard clarifies the wording:

a variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage

with no mention of the nonsensical concept of references declared const.