How this definition is relevant is that the word is used specifically for features that are looked down upon as being harmful in some way, and so using them in programs is creates some kind of risk in addition to "this won't build when it goes away".

The dictionary meaning which comes into play in the software maintenance use of "deprecated" is that "disapprove/disparage" one.

Something that will be removed because it is unfashionable, and being replaced by shiny new idioms, is "obsolescent". Rather than abruptly removing such features, we phase them through an obsolescence period.

Deprecated example: the C gets function (almost impossible to use without creating a security hole, now removed from ISO C).

Obsolescent example: bzero function. (Appears in tradtional BSD sockets networking code, superseded by memset).