@dayue1984, a workaround is to create a notebook called 
Trash (the optional icon can be added by right-click/edit on the notebook name with Joplin version v2.6.2+)
Now just move notes—instead of deleting—to the Trash Notebook, so they remain available to refer to, or move elsewhere, in the future.
You can streamline it further using the Quick Move Plugin which will let you quickly move a note to any notebook by use of a shortcut key.