Well, you can always have repairs blocking the cycleway-side of a shared-use path over several months. That doesn’t make it a footway. Designation is about laws and signage, but not about usability. Which is exactly why these 2 things should be separated, or as @Tolstoi21 said:
=designatedprecisely provides a third option between the two.
The question is, and that’s what @Tolstoi21 says as well, what exactly means “designated” for OSM, and why would we want to capture it? And the simple answer is: ask 2 mappers, and you will get 10 answers.