I would say (not being a developer for QGIS) that the only release to be considered beta would be the master builds of QGIS (ie, nightly).
QGIS (long-term release) can be considered static (no new features added) but with cherry-picked patches that are back ported for stability, enhancement and security.
QGIS (current release) is mainly different from the LTR, that in addition to back-ported patches, it'll get new features added (from master version) when they are considered stable enough with frequent releases (proposed every 3 months).
QGIS (master) is the development version of QGIS which I'd say is beta because when you build it daily there are new features that may be incomplete or existing features may become broken. Not intended for production use.
QGIS LTR and QGIS Current Release can be used for production use.
So to decide, it really depends on whether or not it's a priority that you're looking to:
- maintain a GIS with minimal changes and stability through consistency
(ie, long-term release)
- explore and integrate new functionality from QGIS into your workflows (long-term release, master)
- preparing developed applications and plugins for backwards and future compatibility (long-term + current release, master)
- testing new features (master)
But to answer your question around security... That's tricky because besides QGIS with it's own many potential use-cases and vulnerabilities there's also a long-tail of dependencies that have to be considered as well. For example, one might be in certain scenarios where the versions of QGIS they're using have a dependency mixup like qt4 or 5, python2 or 3, GRASS 6 or 7, etc that would also have to be taken into consideration for security.
Depending on the operating system, package updates for both the long-term and current release may not be frequent enough to give the patches they fix justice (remember it's volunteer based). I've been able to make use of the development cycle by compiling QGIS on Linux directly from the specific master and release branches off of GIT as needed.