Currently usage of phab-ban tool are logged in https://wikitech.wikimedia.org/wiki/Tool:Phab-ban. It is much more appearent than (for phab admin only) directly disable an account (which will leave no public log at all, see another task I will file later), but the log is not fully self-explanatory - so I propose that users can (and should) provide a reason to disable user using the phab-ban tool, and the reason will be logged in that log in wikitech page.
See: T102576: Log user account deactivation also in the feed of the acting admin account
See also T200856: Publicly log account bans made using the phab-ban tool which introduced the public log