Once released, the release enters the stable release process at which time a person is assigned as the maintainer for that stable release. This maintainer monitors activity for the release by investigating and handling nominated patches and backport activity. Only fixes and enhancements that have first been applied on the "master" branch (i.e. the current, in-development branch) are considered for backporting to a stable release.
Stable release branches have strong maintenance for about a year after their initial release. Should significant issues be found for any release regardless of its age, fixes could be backported to older releases. For issues that are not backported given an older release, Community LTS trees and branches exist where community members share patches for older releases. However, these types of patches do not go through the same release process as do point releases. You can find more information about stable branch maintenance at https://wiki.yoctoproject.org/wiki/Stable_branch_maintenance.