In the 7.0 LTS release announcement, it lists “OCI support” under the “Changes since Incus 6.0.6 LTS” section.
It does also say "We have been pretty aggressively backporting changes from the Incus 6.x releases back to the 6.0.x LTS releases" but I don’t think OCI was one of those.
You’re right that a lot has been backported. I think that’s less than ideal: a true LTS branch would be primarily bug fixes, and every backported feature is a potential regression.
On the other hand, the packages in Debian/Ubuntu main repos are essentially frozen in time, which is worse.