(Apologies if this has been addressed before; I did a search but I couldn’t find anything).
I’m unable to update TrueNAS Core in the normal manner, because https://update-master.ixsystems.com/ is currently blocked for me by the (UK) National Cyber Security Centre’s PDNS (Protective Domain Name Service).
I contacted PDNS support. The block resulted from a false positive associated with link[.]storjshare[.]io. The relevant domains are now unblocked, and TrueNAS can “see” updates again.