They are still a standard for Docs, if you look at z/OS docs today, they put all Linux distro and even Kernel docs to shame to be perfectly honest.

Organized PDFs of the order of a thousand pages for literally every subsystem or additional program etc. Over a hundred PDFs in total. Probably 100k+ pages of raw docs.

And that's not even including the Principles of Operations (the mainframe s390x ISA reference).