Synology DSM is based on their own custom Linux. Nothing's changed there recently, except that with 6.x released last year, btrfs became the default file system (used to be ext4).

My mistake then. One of the small NAS vendors moved to FreeBSD. I for the life of me can't think of the name of the vendor.

Don't think so. I maintained multi-vendor NAS'es and always try to be up-to-date.

Do you mean QNAP? Last year they started offering new enterprise NAS with custom OS built on top of BSD kernel

> QNAP Enterprise Storage(QES) operating system, which is based on the simple and efficient FreeBSD kernel and the ZFS file system

https://www.qnap.com/en/product/model.php?II=246

https://www.ixsystems.com/freenas-mini/ ? but I think they've been using it for quite some time.