There is a polite one at the top of Porlock Hill which says "Cyclists are advised to dismount". I chose not to take the advice 41 years ago, and didn't again last summer. I presume that's there as a bit of ar5e covering in case someone goes too fast and can't stop at the bottom. Surely though it's up to any rider to judge their speed and competence on a downhill for themselves with or without a sign? What's the point?
Porlock hill will be well outside the bounds of what most cyclists are used too on a regular basis, so I don't think this is unreasonable here (thinking of inexperienced cyclists in particular).
When leading a club run, I have advised people in the group about the descent of White Down in the North Downs as it is an 18% gradient with a sharp bend right at the bottom of the steep section which you can't see properly until you get close to it.