Zephyr creates a issue Type TEST in IssueTypes .
And also creates a MENU for Test and provide functionality for Test Execution on each project.
When you delete or uninstall the Plug-in, your data against Test will be available in JRIA System. But "Zephyr Test Step" field will be removed from system. And also the Tests MENU on Top along with Tests menu on project.
You would need to back up all the data associated with Zephyr (tickets and test cycles). The test cycle data would be inaccessible and I would suggest porting the tickets to a different issue type that is not utilised by Zephyr. Once it is deactivated the data will not be accessible.
Hope this helps