You can create as many iMovie libraries as you like, keep them all on your Mac or an external drive, and each library will be independent of the other. By holding down the Option key while you open iMovie you can choose from the dialogue box the library in which you want iMovie to open. You can move or copy projects and events from one library to the other. Deleting any one of the libraries can be easily done by moving the library's icon to the trash. Deleting a library will not change the work flow of iMovie, except that you will need to operate in a library other than the one you deleted and that is no longer available. However, when you delete a library, as you have found, it permanently removes from your Mac all projects, events and media that was contained in the deleted library. That is why it is important to store your original media in a place additional to iMovie. Projects, however, can only be stored in an iMovie library, so you would need to have a backup of your library or create a new library to store the desired projects.
Here's a link to Apple's online help menu that explains how to work with multiple libraries:
https://help.apple.com/imovie/mac/10.1/#/mov3fa25bae7
EDITED BY POSTER: You can delete all iMovie libraries and the next time you open iMovie it will create a new blank library in which to open. All of your projects, events, and media that were contained in the deleted libraries will be lost, however. For that reason, I would avoid getting into the habit of deleting iMovie libraries. It is easy to store them on an external drive that is formatted Mac OS Extended (Journaled) or APFS.
-- Rich