Lotus Expeditor is still the underpinning of the Notes Client and Domino Designer. Available literature says it is needed to create a Domino Designer plug-in. So just like the C-API, a version of Lotus Expeditor should be available.
If you search current Domino 12 documentation, you will fin numerous mentions of how the Expeditor toolkit is required for certain functionality.
Just thinking "Expeditor 6.2.1 was created for Notes 8.5 so it is old and can be discarded" is not correct if you are still using the same basic backend, albeit an updated version, since a newer version of Eclipse is used since Notes v9.0.1 FP10. An updated version should be published. That IBM neglected to do this is not a valid reason.