First of all it is not good practice to trigger such back-end events in the front-end, they should be signaled from the API, in this case by the user_delete() function.
If you actually look there, you will notice that MantisBT already defines EVENT_MANAGE_USER_DELETE for this purpose (see documentation), since 1.3.0.
Your install instructions state that a new event
EVENT_ACCOUNT_DELETED
needs to be added in 2 placeshttps://github.com/mantisbt-plugins/Userdata/blob/3e8c2bf3b2ad8e5e7125c44486cf55620db4ee23/docs/install.txt#L13-L28
I do not understand this requirement.
First of all it is not good practice to trigger such back-end events in the front-end, they should be signaled from the API, in this case by the user_delete() function.
If you actually look there, you will notice that MantisBT already defines EVENT_MANAGE_USER_DELETE for this purpose (see documentation), since 1.3.0.