Deleting an m1 project causes this error:
javax.jdo.JDOUserException: One or more instances could not be deleted NestedThrowables: javax.jdo.JDODataStoreException: Delete request failed: DELETE FROM BUILDDEFINITION WHERE ID = ? NestedThrowables: SQL Exception: DELETE on table 'BUILDDEFINITION' caused a violation of foreign key constraint 'BUILDDEFINITCE_FK2' for key (2). The statement has been rolled back.
I think this is somewhat similar to the problem when deleting the default project group. see