absolutely a hybrid. there's a lot of people that are against it and some are for it ...touchy subject for some!! Personally, Scott Felzer and I don't do it, but there are some wicked looking kingsnake hybrids!!!!
Yes anytime you breed different species togeather it is considered a hybrid.
Whether you are talking corn to king or l.pyromelana to l.zonata or t.sirtalis to t.radix they are all considered hybrids. Some hybrids crosses like corns and kings are obviously not as closely related as crosses between members of the same genus like the thamnophis cross you are talking about.
You should note that crossing two subspecies of the same species is not considered hybridization and such crosses generally happen in the wild where the subspecies meet anyway creating areas of integration.
so crossing t.s.sirtalis (eastern garter) to t.s.parietalis would not be considered a hybrid because they are both in the sirtalis species and both likely integrate where their ranges meet anyway.
Purist breeders look down on these types of crosses as well. Some even look down on breeding animals of the same subspecies from different localities creating "generic" specimens vs. "locality specific".
ThamnophisWest.com - Breeder of select garter snake species from the western United States.