The DB2 CLI runtime environment is included with any of the DB2 Client Application Enabler products. Development support for each platform is provided by the corresponding DB2 Software Developer's Kit (DB2 SDK) which is part of the separately orderable DB2 Application Development Kit product.
For example, OS/2 applications are developed using DB2 SDK for OS/2, and can run against any DB2 server using DB2 Client Application Enabler for OS/2.