once 7.6 is out, 7.5 is dead: we are using the concept of “rolling release”, so old releases should be always updated to latest one. Also, old releases will not get any update (including security fixes from upstream)
supporting the same software on multiple platforms quickly become a mess
The CentOS Project provides updates or other changes ONLY for the latest version of each major branch.
Thus, if the latest minor version of CentOS-6 is version 6.6 then the CentOS Project only provides updated software for this minor version in the 6 branch.
If you are using an older minor version than the latest in a given branch, then you are missing security and bugfix updates.