ZBDs relay doesn't do anything with your NSEC.
Discussion
Are everybody here talking about some specific client's relay, nostr:npub18ams6ewn5aj2n3wt2qawzglx9mr4nzksxhvrdc4gzrecw7n5tvjqctp424, please?
They don't but do they let you edit your relay list? If you want to move your nsec to another client, you're stuck.
You don't have to use the ZBD app to use the ZBD relay. I was using it for many months. It was one of the more popular relays for a long while.