See https://gerrit.wikimedia.org/r/#/c/264718/ for an example of a use case.

Brainstorming:

• Add ApiBase::PARAM_VALUE_DEPRECATED as a string[] naming the deprecated parameters.
• ApiParamInfo would want to include this in its output.
• In combination with ApiBase::PARAM_HELP_MSG_PER_VALUE, ApiHelp and ApiParamInfo should indicate this in the message text.
• Would there be a sane way to indicate it in the "list of possible values" too?
• Parameter type 'submodule' should ideally pick this up from the submodules' isDeprecated() methods, but be careful of performance. See also T123930.