Does Frequently used Memoji History reset If not used
The Memoji stickers can be deleted in any app, tap the three dots, then tap the core Memoji and delete it. I’ve never seen them time out or go away, just new ones pile on top of the old as you use them. 



Tap the Memoji identity you with to duplicate or delete.



As for the overall keyboard, you can reset all of the word suggestions, etc in the settings app. But you circled the Memoji which just exist until you delete the identities in the keyboard overlay itself. 


Settings > General > Reset > Reset Keyboard Dictionary