If you stop a trial of draw.io for Confluence Server, increase the Confluence user tier without increasing the tier of draw.io, or use a draw.io version released after your maintenance expired, draw.io will be no longer licensed.
The draw.io diagrams are stored as attachments to the pages you inserted them on, that's the only place they are stored and they remain regardless of whether the plugin is licensed.
Without a license, diagrams will continue to be viewable in Confluence, but not be editable. Note that you hold your data in the host and draw.io is an open source application, so you can download the attachment and edit/save the diagram using the free online version or the free desktop version.
If you uninstall the plugin, the attachments remain in place, but Confluence will not know what to do with the attachment, the viewer in either product will not function.