OS X Server is no more. It was already a zombie, but still, it's official. Is there a free, reliable, directory service with a GUI? I'd probably run it for home stuff at the very least.
It started dying back in 2009 but when in 10.6.8 Microsoft Server 2010 natively allowed Mac OS X and Linux machines to connect to Domain server and be managed like a regular computer! Mac OS X sever started its slow death spiral! However today smart NAS boxes have become so good they now can setup a Domain Sever so Mac Server has depreciated!
An old Mac set up with LINUX would probably serve best for many. If you just want to share then Monterey seems to have moved on from the problems of Big Sur and Catalina. Server wasn't very cloud-centric or suitable for remote working so I suspect the drift is towards these systems. See https://support.apple.com/en-gb/guide/deployment/dep1d7afa557/web of which Apple have their own.
Synology has a directory server...that I hear is OK for small orgs. Have not used it. I saw somewhere that it is essentially Samba 4 AD. Apache might be worth a look too. No idea of risk vs reward...but I understand Google has an option too, for certain types of customers.
Good overview stuff here...might be useful for those working out what is next.
The free core version of univention looks promising...on Linux. I see they have VM images too, which could be fun.