Used in lots of places if they don't want to go GPFS, Lustre, maybe CephFS nowadays. Dell-EMC Isilon is used in lots of places for NFS (and SMB): I worked at a place that had >10PB in one file system/namespace (each node both serves traffic and has disk/flash, replicated over a back-end).
> […] we use a centralised FreeIPA server that allows all machines in the cluster have the same UID/GID mapping everywhere.
(Open)LDAP is still very handy as well and used in many places. (AD is technically LDAP+Kerberos.)