You can still add people with your old Username#0000 even if you got the pop up.

I noticed that you can still add people with there old username with the old #0000 shown here

This is concerning as you are unable to change the old username if you are being harassed online. While yes people can disable friend request from unknown people they can get your current username easily due to this. The fact that the Old username is Still active even though you have changed Should not be possible

No i dont care if people know my username, mods i own the accounts that are shown above and dont mind outside friend request if you are going to delete my post. I am willing to show proof if needed*