A New Meaning of 'Deprecate'
Update: This meaning was added in June 2018.
Deprecated is increasingly used as a technical term meaning "to recommend against using something on the grounds that it is obsolete," or "to declare some technological feature or function to be obsolescent." It's another example of technologists borrowing an existing word to describe something new, just as mouse was borrowed to describe the device that moves a cursor. Yet the word deprecate has already shifted its meaning in ways which make some people uncomfortable—and it's unclear whether the confusion surrounding this commonly confused word has contributed to its new technical usage.
The earliest meaning of deprecate was "to pray against, as an evil," and soon after this first meaning it took on the additional sense "to express disapproval of." Meanwhile, depreciate, the closely related word with which it is often confused, means "to lower in value."
Although deprecate retains its meaning of disapproval, it has also come to be employed in a fashion that is similar to a figurative sense of depreciate (more "to belittle" than "to lower in value"). This can be seen in its appearance in self-deprecating—which originally was self-depreciating.
The fact that usage guides have spent a number of decades explaining to people how they should not use deprecate appears to have had little influence on the 1980s computer users who first used the word to describe obsolescent technology:
In the BUGS subsection, the alias facility is deprecated vs. the use of Shell files.
(Jack K. Cohen, Critique of 4.2BSD Documentation (Vol I), August 2, 1984)
Perhaps the standard should require minimum 6 char caseless externals, but the implementation of anything less than arbitrary length case-distinct is, as they say, 'deprecated'.
(Kevin Martin, 6 char externs and the ANSI standard, November 17, 1984)
These technologists used deprecated in a very specific fashion, and it is clear that it has a meaning for which there is no other suitable word. It is less clear, however, exactly why deprecate was chosen over some other existing word, or instead of creating a new one. One possibility is that they associated it with the "to lessen in worth" sense of depreciate. Perhaps a clue to another possibility may be found in the writings of one of the word’s early adopters, Kevin Martin (quoted above). After using deprecate he attached a definition from The Random House College Dictionary, made note of that book’s fourth sense—
4. (archaic) to pray for deliverance from
—and added “I often find the 4th definition appropriate :-)”