Junos version upgrades are provided for devices until the end of support date listed on the site.
There are no separate security patches. All Junos changes, bug fixes, security updates and added features are rolled into new Junos releases. These increase the sub version number as you go up the chain.
Security bulletins are posted and list the version where the issue is addressed.
https://kb.juniper.net/InfoCenter/index?page=content&channel=SECURITY_ADVISORIES
Bugs are tracked in the public PR (problem report) data base where you can search for issues by platform and version. The release notes give the highlights for the various upgrade paths.
https://prsearch.juniper.net/InfoCenter/index?page=prsearch
And the support center, JTAC lists their current recommendation for version by platform.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB19372