The user_delete_clean_steps event passes DeleteCleanUpService, but there is no user's getters, only $userId, $userName and $originalUserName
properties that are declared as protected, inaccessible by event handlers
user_delete_clean_init/user_delete_clean_steps both pass the DeleteCleanUpService in, but while there isn't any public getters it is fairly trivial to bind a closure to the DeleteCleanUpService class context to manipulate internal state if you really needed to.
The argument order and suggested type hints means you can't really remove that argument without breaking backwards compatibility.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.3.11).
Change log:
Add public getters to DeleteCleanUpService for code event listeners