There is no widely used custodial nsec bunker, the popular ones are Amber and nsec.app both non-custodial.
Discussion
nsec.app is basically third-party custody
Isn't the keys on the device? It isn't uploaded to the server. So why do you say that?
The only thing that's truly not third-party custody is a good hardware wallet used properly
But nsec.app is especially "bad" (though not useless) because it would be so easy for it to steal nsec keys without detection