Lookup options include
[✓] Required (Always require a value in this field in order to save a record)
What to do if the lookup record is deleted?
( ) Clear the value of this field. You can't choose this option if you make this field required.
(•) Don't allow deletion of the lookup record that's part of a lookup relationship.
Once a custom lookup field is packaged, can those options be changed in subsequent releases?
If required is removed, package is packaged again, and an upgrade is performed, will the custom lookup field no longer be required in orgs that are using the upgraded package?