I have read where Apple plans to stop making OSX server. What does this mean for LDAP services? Is it possible to install LDAP on a regular mac running High Sierra? Could my own iMac be the LDAP server for my organization in other words?
macOS Server is keeping LDAP with no published plans for it to be removed. See:
To answer the question(s)
Is it possible to install LDAP on a regular mac running High Sierra?
Could my own iMac be the LDAP server for my organization in other words?
Yes on both counts, though I wouldn't put LDAP server on a production workstation (if you turn off your iMac, LDAP goes down for everyone) You could put it on a Mac mini (two would be better) and let it run as server 24/7.
LDAP is just a service much like SSH, Apache, or DHCP. You don't need a server version of an OS to run a server instance.
OpenLDAP is available via Homebrew and you can run this on your plain macOS machine.
That said (non-Apple solution follows) I probably wouldn't run LDAP on a Mac - it's just too expensive (hardware wise). I would get two cheap PCs, load up FreeBSD (what Mac is based on) and fire up OpenLDAP on each.