Obsolete Functions

You can mark a named function as obsolete, meaning that it may be removed at some point in the future. This causes Emacs to warn that the function is obsolete whenever it byte-compiles code containing that function, and whenever it displays the documentation for that function. In all other respects, an obsolete function behaves like any other function.

The easiest way to mark a function as obsolete is to put a (declare (obsolete ...)) form in the function's defun definition. See Declare Form. Alternatively, you can use the make-obsolete function, described below.

A macro (see Macros) can also be marked obsolete with make-obsolete; this has the same effects as for a function. An alias for a function or macro can also be marked as obsolete; this makes the alias itself obsolete, not the function or macro which it resolves to.

— Function: make-obsolete obsolete-name current-name &optional when

This function marks obsolete-name as obsolete. obsolete-name should be a symbol naming a function or macro, or an alias for a function or macro.
If current-name is a symbol, the warning message says to use current-name instead of obsolete-name. current-name does not need to be an alias for obsolete-name; it can be a different function with similar functionality. current-name can also be a string, which serves as the warning message. The message should begin in lower case, and end with a period. It can also be nil, in which case the warning message provides no additional details.
If provided, when should be a string indicating when the function was first made obsolete—for example, a date or a release number.

— Macro: define-obsolete-function-alias obsolete-name current-name &optional when doc

This convenience macro marks the function obsolete-name obsolete and also defines it as an alias for the function current-name. It is equivalent to the following:
          (defalias obsolete-name current-name doc)
          (make-obsolete obsolete-name current-name when)

In addition, you can mark a particular calling convention for a function as obsolete:

— Function: set-advertised-calling-convention function signature when

This function specifies the argument list signature as the correct way to call function. This causes the Emacs byte compiler to issue a warning whenever it comes across an Emacs Lisp program that calls function any other way (however, it will still allow the code to be byte compiled). when should be a string indicating when the variable was first made obsolete (usually a version number string).
For instance, in old versions of Emacs the sit-for function accepted three arguments, like this
            (sit-for seconds milliseconds nodisp)
However, calling sit-for this way is considered obsolete (see Waiting). The old calling convention is deprecated like this:
          (set-advertised-calling-convention
            'sit-for '(seconds &optional nodisp) "22.1")

13.12 Declaring Functions Obsolete