Why, just above!
I can't think of any specific thread, but whenever anyone mentioned xml versus openstep, one or another member of the dev team - and sometimes 2 or 3 of us at once - does tend to mention something along these lines: yes, xml is the newer format, but openstep is easier to use & read, so we'd use that instead.
For this very reason Oolite's internal plists have been fully converted to openstep about a year or so ago ( I think: it definitely happened a while before lucid and its buggy .plist parser got released...
)
Somewhere or other, on this forum, there was also some mention of updating the wiki - regrettably a lot of examples are still in xml as opposed to openstep - but unfortunately RL has been a bit too demanding for a lot of us...
PS: xml is still very much supported by the basic environment (gnustep & cocoa - the unfortunate snafu with lucid notwithstanding), so if you really, really want to use that as opposed to openstep format, by all means, do so!