There are many tables that lack explicit foreign key relationships to parent tables.  For example:

* serial (biblionumber, subscriptionid)
* subscription (biblionumber)
* subscriptionhistory (biblionumber, subscriptionid)
* suggestions (suggestedby, managedby, rejectedby)
* branch_transfer_limits (toBranch, fromBranch)

and many, many others.

Explicit foreign keys should be added for several reasons:

* to promote better data integrity and to expose cases where the application code makes unwarranted assumptions about the database structure
* to allow cascading updates and deletes in some cases
* to make it easier for external reporting tools to correctly identify relationships between tables
* to make use of an ORM more effective

There are some arguments against this:

* a complete set of FKs may decrease database speed (on the other hand, the Koha schema already uses a bunch of FKs)

There are some implementation details to consider:

* the database creation scripts may need some reordering
* in some cases, a database update to add an FK may require dealing with rows that do not meet the new constraint.