I have radicale installed on my server with a self-signed certificate. It stores data in standardized clear-text files and has good documentation on how to setup all sorts of clients.
I was only six at the time, so I don't have first hand knowledge, but from what I gather there was no really free kernel/operating system at that time. Minix was under license restrictions, the BSDs was tied up in legal battles, and GNU was still on it's endless search for the perfect kernel.