If you run JES3, or if you know someone who does, please read on for some important planning information.
For decades, z/OS and its predecessors offered two job entry subsystems: JES2, included at no additional charge, and JES3, available as an extra cost option. There's a long, complicated development and marketing history associated with both subsystems, but I'll skip that history on this occasion.
Fast forward to 2017, when IBM announced that JES2 is its "strategic" offering. That important word means IBM plans to deliver enhancements and improvements to JES2 exclusively. IBM has already substantially enhanced JES2 to make it easier to migrate from JES3 to JES2. Earlier this year, IBM announced that the z/OS release following z/OS 2.4 would be the last release to offer JES3. For sake of argument let's call that future release "z/OS 2021," with the assumption (but not ironclad promise) that IBM will make that next release generally available in September, 2021. IBM's current practice is to support each z/OS release for 5 years and then to offer an optional service extension (at an extra cost) for up to 3 years. Add that all together and, for planning purposes anyway, IBM will likely support JES3 until September 30, 2029. To get JES3 support from IBM until that date you'd have to run "z/OS 2021" and buy a maximum length z/OS service extension, with the key assumption (but not ironclad promise) that IBM's plans and support practices are at least as favorable as today's. Past that estimated date presumably you could continue licensing and running JES3 from IBM but without IBM support.
"IBM encourages you to start planning your migration" from JES3 to JES2, and you can contact IBM via e-mail at email@example.com for specialist advice. It's generally not too hard to migrate, and you'll be able to drop your JES3 license and associated charges after you complete your JES3 to JES2 migration. Many z/OS sites have successfully migrated.
However, if you expect you'll absolutely need to run JES3 beyond 2029, with support, there's another possible choice. Phoenix Software International and IBM recently reached a JES3 technology transfer agreement, and Phoenix Software plans to introduce its own job entry subsystem product in early 2020 based on IBM's JES3 source code. This choice somewhat reminds me of MacKinney Systems' CICS compatibility add-on modules such as their Macro Level Interpreter and VS/Cobol Interpreter.
If you have some thoughts to share about JES3 and your plans, please post a comment.