It never was supported. Just now you will get an error instead of wrong result when you use BooleanOptionalAction in the way it was not supposed to be used.

Right, but isn't the text I mentioned above suggesting that --no-foo can be used with BooleanOptionalAction? After this PR is merged, this is no longer recommended, as you'll get an error, so shouldn't we remove this as an example in the documentation?

but isn't the text I mentioned above suggesting that --no-foo can be used with BooleanOptionalAction?

Read it in the context. The following example shows that it creates action for --no-foo when '--foo' is passed to add_argument().

Yeah, I see the example. It just sort of reads a bit unclear to me and implies that I should be able to create a parser with:
parser.add_argument('--no-foo', action=argparse.BooleanOptionalAction), which is no longer the case. My comment was more about creating clarity in the docs that --no- prefixed options are not supported.

I do not see anything that implies that '--no-foo' works. And it never worked.

Thank you for your review @savannahostrowski.