The initial issue comes from earlier implementation that exposed the getLineSpacing method as an inherited public member instead of using composition to hiding it. This method was introduced in M5. Although it wasn't annotated as @noreference and public, this method was never meant to be part of the API and we are not aware of anyone who's referencing it from InlineAnnotationSupport nor of any use case for which anyone would want to use it. So we feel confident that it's a safe thing to get rid of it now.
PMC, please evaluate whether changing this API is acceptable at this point and mention ASAP if and when we can re-apply the suggested patch.