Koeberg Nature Reserve is open as of 1st of March 2023. It will only be accessible for OCA permit holders. Fortunately, it's not hard to apply for a permit. When you come in via the R27 entrance, you can tell security you want to apply for a permit.

This permit is issued on the second building, the FFD centre, on the left. FFD Centre is open from 14:00 until 16:00 Mo-Fri for processing of these permits. You need to fill in a form and off you go. 

Thanks everyone!