See discussion at #9685 (comment) and #9685 (comment).

The first step is figuring out how to get Sphinx to properly render the deprecated directive when it's in the args list. To do this, use a repo like qiskit-ibmq-provider (because it's small) and manually add this to the docstring in the Args list:

.. deprecated:: 9.99
  Deprecated!

Make sure Sphinx builds and check the generated HTML to ensure everything renders properly. Make sure this works both when the arg has no content already and when it does. Experiment with different types of indentation that developers may write.

Then, once you figure out how to properly add the directive, refactor the logic in deprecation.py to insert the deprecation in the Args list, rather than in-between the summary and the metadata lines. Make sure to handle indentation properly, and use test driven development (TDD) in test_deprecation.py.

Also, figure out how to handle when the argument is missing from the Args list or there is no list. Should we dynamically create the section? Add it to the section? Or, stick with rendering it in-between the summary and args list (I vote this).