You cannot find Party Role in the Code List Restriction for Leads.
Reproducing the Issue
1. Go to the Administrator workcenter.
2. Go to the General Settings view.
3. Click on Code list restrictions.
4. Click New.
5. Select the Business Object Lead.
Result: Party Role is not shown in the field Code to Restrict.
This is the expected system behaviour. Party Role is not available in Leads.
Not all Business Objects offer the same restrictions.
Having the Party Role restriction in Leads would be a new requirement.
KBA , AP-CR-CR , Customer Requirement , Product Enhancement