Verizon didn't lock their phones back when the S6 was still available.
If there is a lock of some kind on it, there are only 2 scenarios I can think of.
Either it was sold as a prepaid phone in which case it is locked to Verizon prepaid for a certain duration.
Or the phone was bought on device payments and never paid off, in which case it's legally a stolen and blacklisted phone.