Actually, TrueOS is tracking FreeBSD-12CURRENT and to the best of my knowledge, that’s what it’s going to track. Not 11-Release or 10-Release. Not sure what is going to happen when 12-CURRENT becomes 12-Release but that’s a little ways off now.
The TrueOS STABLE release is on a 6 month update schedule, so while it tracks 12-CURRENT, it is not changing rapidly. Packages for TrueOS STABLE are also locked down for the 6 month period.
TrueOS UNSTABLE is also based off of 12-CURRENT, but updates on a more frequent schedule, roughly every 2 weeks. Packages in the UNSTABLE repo are locked for that period.
A new TrueOS UNSTABLE release may or may not include updates from FreeBSD proper; the devs in charge of the TrueOS repos (base and ports) keep an eye on commits to FreeBSD so they don’t give us a broken UNSTABLE.
Just because there is an update available, one does not have to install it. If there is an ABI change, one really should have new packages (ports) installed so everything stays consistent.