Hi Steven,
This is what I can see "You archive an event handler that belongs to one or more event handler sets by deactivating and archiving the entire event handler set" and check in this link Archiving Event Handlers (SCM-EM) - Archiving in SAP Event Management - SAP Library.
It will have some implications, if we deactivate particular event handler in event handler set then subsequent event message may fail for other event handler in EH set, SAP EM Web UI doesn't retrieve event handler data as other event handler deactivated in EH set etc. You can raise SAP OSS message to SAP to get the list of causes deactivating event handlers in EH set.
Regards
GGOPII