There are no more pkg builds for FreeBSD 11.0. You could probably do your own, and it may work, but I think your best bet is to move to FreeNAS 11.2-U1(!), then 11.2-U8, then recreate the Plex plugin as iocage (it's warden right now), then move to FreeNAS 11.3-U4.1.
From U1 you can likely just use the in-UI upgrade methods (you'll need to be on U8 after U1 to recreate your jail), but in case you want the ISOs:
Make sure to have snapshots before you start, understand boot environments, read up on iocage, and if you are booting from USB, switch to an SSD (possibly as the "M.2 2242 in USB stick" doohickey that people on these forums are using) as a boot device so the USB stick doesn't die on you during upgrade.
Currently not really having any issues, but I do have a dilemma that’s I was hoping some more intelligent than I could offer some proper guidance on. Originally I had a single 16GB USB Cruzer running freenas and I have since added an additional mirror after seeing some weird read errors. I...
www.ixsystems.com
And consider using danb35's script to create your Plex jail. You can also bring all your metadata over. If you're going that way, here's YT that describes the process. Ignore the hw transcode bits, that's outdated now.
https://www.youtube.com/watch?v=GvM32lWc_r4
With TrueNAS Core 12.0, there is now an official Software Development Release Cycle, documented at
https://www.truenas.com/docs/hub/intro/lifecycle/ . This means that currently, 11.2 and 11.3 are supported, and come October, 11.3 and 12.0 will be supported. Once 12.1 is out, 12.0 and 12.1 will be supported.
At the current rate of development, that does mean upgrading every two years, maybe even every year, to a new major release. My experience since 11.2-RC1 has been that this goes smoothly, though it can be wise to wait until the new release has had a little time to "bake" before making the switch. 11.3-U2 had a nasty bug, for example; and 11.3-U4 an annoying one.
I expect FreeBSD to stick with iocage jails, I don't think there'll be another transition anytime soon. My crystal ball may be faulty, of course.