Yes, you need to rebuild them, after being upgraded to Mx9.
Yes, you can still develop them.
See https://docs.mendix.com/refguide/mobile/introduction-to-mobile-technologies/hybrid-mobile/ for Mendix’ take on it. So you best migrate/rebuild your app in any of the available other profiles, but if you have reasons to keep them in Hybrid, Mx9 will allow you to do so.