Ummm... You never had the option to opt out. Your nostr keys were always Bitcoin keys. They're derived with the same cryptographic primitives. Amethyst and Ditto just eliminated the barrier of having to write a custom script to convert the format, but people have been able to send Bitcoin to your npub for as long as nostr has existed. Nobody however is forcing you to move the funds to another wallet linking identities as a result. You can just leave it alone. That's the choice you do have.