Paper Essentials
The original tools were sold as Paper Essentials until February 2015 when the tools were made free for all.
Your previous purchase of Paper Essentials should enable you to access to the Diagram, Cut, and Fill tools in Paper 5 if you're signed in with the same Apple ID. Tap on the Settings icon top right, then "Support" and on the next screen tap on the "Restore Purchases" button.
Legacy accounts
Paper users who may or may not have created a Paper account before November 30, 2017, were considered to be Legacy users. Legacy users had access to the Diagram, Cut, and Fill tools for free.
Please note that if you were one of these Legacy users, reinstalling the app or changing devices may result in the loss of Legacy status. Once Legacy status is lost it will be necessary to subscribe to Paper Pro to unlock the tools.