That prefix tells you where users are from and not all users would agree with your request so it should be best implemented as something that can be optionally turned on in the app settings instead of being hard coded into the app
And it would be pretty hard to mention a user on lemmy if someone doesn't know the instance that user is from
Because it is as follows: @x4740N@lemmy.world
To mention a user and you obviously need to know the instance a user is from
I do agree with cleaning it up a bit if it needs to be but I don't personally use the app and am just commenting my opinion on this post
But it shouldn't be outright removed