I have an older copy of Volume Four of "The Definitive Guides to
the X Window System", which is subtitled "X Toolkit Intrinsics
Programming Manual" (Second Edition, published 1990).
It has a bunch of ICCCM atoms for selection in section 10.2.9.
These atoms include 'TARGETS', 'MULTIPLE', and 'STRING',
which AFAIK are still supported today, but it also contains
atoms such as 'ODIF'.
ODIF references the ISO Office Document Interchange Format
(which apparently is RFC1341 -- MIME). A simple test
tool indicates that neither Abiword nor Galeon (two fairly
popular Linux applications) appear to support this atom.
Neither does Mozilla.
I should note that the copy I have is too old to mention 'INCR',
which was used by Tk's Text widget at one point for incremental
selections. (I don't know offhand whether it's used by other
widgets, although one would hope so.)
So now I'm curious; the dumb question of the day is: was 'ODIF'
or the entire X select paradigm replaced, and if so, by what?
(The cutbuffers of course are a different issue.)
It's still legal to go .sigless.