@climagic another great part of documentation is this section in the xorg.conf manpage.
there is probably a huge amount of X code/features which aren't used much anymore (e.g. features which aren't used anymore (as there are X entensions for these) or user interface rendering in the gui toolkit passing bitmaps instead of using the drawing commands in X)
I think quite a few X developers noticed this themselves and are now working on wayland instead (see for example this wayland talk by a former X Developer: https://youtu.be/RIctzAQOe44)
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