Is a non-restrictive relative clause a subordinate clause?

a. The mayor, who lives in this house, has not been seen for days.

This is a non-restrictive relative clause, since it provides supplementary information about the mayor, but is not essential to the sentence – if the clause were omitted, it would still be known which mayor is meant. If the bold part is deleted the remaining part provides the sense. (Wikipedia)

Is a non-restrictive relative clause such as this a subordinate clause?

"A nonessential (nonrestrictive) clause is a subordinate clause that is not essential to the meaning of the sentence but merely adds an idea to the sentence." [1]

Thus, by definition, a nonrestrictive clause is also a subordinate clause.

[1]: Warriner's English Grammar and Composition, Complete Course