Re: variable or function for list of known text properties?

[Richard Stallman]

    An Elisp function might want to use a list of such known text properties in
    some way (e.g. as TABLE arg to `completing-read' or in a defcustom :type
    expression). The list of predefined properties depends on the Emacs release
    used.

I do not see why this would be useful.  In any given place, each standard
property probably is or is not appropriate, so you need your own list anyway.