Do note, this is not a forced command, it is optional
People can choose if they want their rank to be shown or not. and in this case, even if they know this was implemented, they would not be so sure if there is any chances that one of the people in the server is ranked.
Of course, unless they doesn't care looking like a fool spamming opopopop without any ranked player in the lobby, that would be a problem, but as far as the "logical" people, they probably won't.
Which brings me back to my earlier comment.
What I assume they will do is just spam opopop in every game they join hoping someone with a rank that's vanished, is here and would choose op.
I'm not saying I disagree with your idea and I'm not saying I have any better suggestion but what would come after that? More spams since they won't know if a donor is in the lobby or not.
What I'm trying to say is that, some players can be very ignorant. If I were an ignorant player, I would spam opopop every time in pre-game lobby or if I see a a donor in lobby, I would follow them in game and spam it either way. So I don't really think it would be efficient.
Yes it is optional, so if my simulation works out and it does back fire then reverted to what we have now, they will just assume most of the donors are hiding their prefix and just spam other way. If you tell them the add-on was removed, they would think you're making some sort of excuse.
They may be logical, they may ignorant.
You can change the game as much as you want but you can never change how someone behaves and trust me, there are tons of players who would spam for the sake of it.