But TCP or HTTP don’t care about the underlying transport. They’re higher level protocols that are payloads to either IPv4 or IPv6. It’s irrelevant what the transport is when dissecting HTTP and very little time should be spent on it.

IPv4 is, for all intents and purposes, still the default transport. It’s also simpler than IPv6 in some regards. When teaching layer 3, it makes sense to teach both, and teach IPv4 first. Though I fully agree that they should be taught with equal emphasis. I don’t doubt there’s a good number of programs out there that don’t into sufficient detail on IPv6.