The -releases are fixed and never change. Updates to -release packages are made in the -stable branch, but only to existing applications when there are CVEs or security-related updates from the upstream projects.
http://www.openbsd.org/faq/faq5.html#Flavors
Edited to add: the project does not build packages from the -stable branch, they are available as ports only. M:Tier offers -stable packages to the OpenBSD community as a service.
https://stable.mtier.org/