The USERKEY parameter establishes control over user resources in the same way that the CICSKEY parameter provides control over CICS resources. Hence, USERKEY and CICSKEY are functionally equivalent. However, the CICS interface defines CICSKEY resources (for example, files, transactions, transient data etc), while USERKEY resources to provide control over your own resources and same keywords are used as explained here.
The CICS interface supplies the USERKEY of RESOURCE=ACF2CTRL already defined to control access to the functions of the ACFM transaction.
If API is used to make a call for user defined resource, that resource class should match the USERKEY.
To identify if the USERKEY is still being used, recommendation is to change the USERKEY resource rules to specify LOG and monitor the ACFRPTRV report for any access being done for these resources.