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).
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