Extension Framework uses attribute to instantiate descendants and you need to flush cache after creating or removing an attribute. There is a blog post how to do this from code, but we have a way to do the same from UI (and it’s quite important because you cannot go and create a job in new AX anymore to flush your cache after deployment).
Go to Transportation management->Setup->Load building->Load building strategies form and click on Generate class list button. Under the hood AX will call SysFlushAOD::main() to clear all caches.
This button will clear various of different caches, not only Extension Framework one, so could be used in different scenarios! Kudos to my fellow colleague Alexey Borisov for pointing this out!