If it doesn’t have to be from F-Droid repo specifically, here’s one maybe. No screenshots so you may have to check it out.
K9Keyboard - IzzyOnDroid F-Droid Repository
Regarding Thumb-Key, not T9 but still pretty cool. On all my devices, forcing myself to stick with one odd keyboard so I can get totally familiar. Fun stuff.
Also regarding Smart Keyboard Pro. Their FAQ seems to imply that PayPal can be used to buy a key but link goes nowhere. The Forum also but really outdated discussion. Maybe you can contact the dev and see if alternative payment methods are still available.