> the X.500 directory system, a system from the 20th century which was never actually deployed

X.500 really was deployed – never at the scale the designers originally intended, as a single global directory system – but, as an enterprise directory system, yes it was – and it still survives in that role today, albeit as a legacy niche.

LDAP is a direct descendant of X.500 – it was basically taking the X.500 Directory Access Protocol (DAP, X.511), simplifying it somewhat, and porting it to run on top of TCP instead of OSI TP. Many early LDAP servers were just X.500 DAP servers with LDAP support added as an additional feature–and if you read the LDAP RFCs, large parts of them were written with that assumption, and don't make much sense unless you understanding the X.500 underpinnings

Nowadays, the most popular LDAP servers never implemented X.500, and few bother to implement the full set of X.500 semantics which LDAP supports – although one of the infuriating things about LDAP is that every implementation is a slightly different subset of the X.500 feature set.

X.400 survives in some applications–it is the basis of the NATO standard Military Message Handling System (MMHS) and also the Aeronautical Message Handling System (AMHS) used by commercial aircraft – and X.400 and X.500 were designed to be used together. I know vendors like Thales Group still sell X.500 directory servers for use with AMHS (and probably MMHS too)

Isode still sells M-Vault which supports both LDAP and X.500 DAP (X.511) – primarily for military applications