Note: This requires enabling metamta.recipients.show-hints (it is enabled by default. Which I believe does not make sense).

Every [plain text] email body (at least for Maniphest, Diffusion, Differential) includes two bottom lines

To: Username1
Cc: Username2, Username3

created in src/applications/metamta/replyhandler/PhabricatorMailTarget.php.

Either its data (the listed accounts) is incorrect, or Phorge sends mail to disabled accounts which I'd also consider a bug.

Downstream example: Some task recently got declined.
An account disabled many years ago is a watcher of the (sub)project that declined task is tagged with.
The notification email which I received about that task getting declined lists this disabled user account in the Cc: footer in the email body.