I had reached out to iThemes about registering for CP. Their response is below:
Thank you for the offer and for bringing this to our attention! Currently, we do not plan to develop anything for ClassicPress. You should still be able to use Security, for now, however we can’t guarantee that it will continue to update as both Security and ClassicPress continue to update.
For now, no plans, but that doesn’t mean they won’t at some point. iTSec does currently work with CP 1. 1 and iTSec 7.4.1