From LQWiki

When a program or feature is considered obsolescent and in the process of being phased out, usually in favor of a specified replacement, it is said to be deprecated. Deprecated features can, unfortunately, linger on for many years. This term appears with distressing frequency in standards documents when the committees writing the documents realize that large amounts of extant (and presumably happily working) code depend on the feature(s) that have passed out of favor. See also dusty deck.

Also, sometimes, the use of an entire software package (or, at the other extreme, maybe just a few library calls) might be deprecated in favor of more modern and current replacements.

[Usage note: don't confuse this word with ‘depreciated’, or the verb form ‘deprecate’ with ‘depreciate’. They are different words; see any dictionary for discussion.]

This article is based, in whole or in part, on entry or entries in the Jargon File.