The bin/remove script is used to permanently destroy data in Phabricator. Normally, this isn't allowed from the web UI for most objects.

If you try to permanently destroy a Passphrase credential:

phabricator/ $ ./bin/remove destroy K123

...you'll get an error that the object isn't destructible. To make it destructible:

• First, rename the interface PhabricatorDestructableInterface (which I have just realized is misspelled) to PhabricatorDestructibleInterface.
• Now, implement PhabricatorDestructibleInterface on PassphraseCredential.
• Look at other classes which implement this interface for examples.
• The only linked object you need to destroy explicitly is the associated PassphraseSecret (if one exists).
• You can use bin/remove destroy K123 --trace to get more information about what the command is doing.