Ship It!

The change as-is looks great.
Many of the changing function signatures should also be made keyword-only while we're changing them. I commented on the major locations. This could all be done separately from this change though.

Can you verify if rbintegrations, Power Pack, and Review Bot tests pass?

While here, can you fix up the ordering of these imported classes?

Given the "diff_settings will be required," and diff_settings being keyword-only, we can remove the None and callers will have to provide it (will need a re-run of the test suite though).

This could also be removed once the above is changed.

We don't explicitly state this (oops), but as the old argument is deprecated, we can remove enable_syntax_highlighting here and probably make diff_settings mandatory.

The docs state that parsed_diff_change is mandatory in 6.0. Since this is a keyword argument, I propose:

Making all arguments keyword-only, and documenting that.
Use the @deprecate_non_keyword_only_args arguments.
Assert parsed_diff_change until 7.

That will also let us remove the conditional below.

While here and impacting keyword argument order, let's make all these keyword-only arguments and use the decorator to deprecate positional use.
As per docs, diff_settings is also now required, so we can remove the None.

This should also be removed.

Since we're affecting keyword argument order now, let's make these keyword-only and add the decorator.

Same comment re: keyword-only arguments and decorator.