I think this is ridiculous.
Let’s imagine this scenario - here is a user with no discourse experience. He/She joins, browses, and the system sends a private message: “Hey! You’ve been promoted!”
This private message will only be sent once. Apparently you can’t be promoted to TL1 or 2 twice.
Therefore, when this user who has no experience in using discourse sees the button “Don’t send me a trust level promotion message”, this user should feel puzzled. No one has ever told him what a trust level is. And once sent, this button becomes useless.
So the only people who need to use this are users who have used several different discourse forums before. How many of these people are there? Is it necessary to develop a function for these people? If necessary, how much functionality should be packed into the discourse system?