I saw a reference to ZFS #16402 in the release notes to 2.2.6. Looks like something to make L2ARC significantly more meaningful for broad use and without much tuning? Only by setting l2arc_mfuonly to 2 (if not default).
Specially the change allows L2ARC to be fed with all metadata but only MFU (Most Frequently Used) data, ie not MRU (Most Recently Used), which should reduce L2ARC churn significantly because it would avoid feeding it with read-once data.
The option was there before also, but with fewer options.
In any case the setting right now in TrueNAS is 0 which means L2ARC gets fed both MFU and MRU for both metadata and data. When ZFS 2.2.6 hits TrueNAS I think a sensible default would be to set this to 2, or at least recommend users to look in that direction themselves. Looks like a great feature.