Some mods can make the game way too easy or itch that power fantasy, make the player more OP, etc....use at your own risk.
But I just can't see the 'messenger' system being a part of those examples or the reasoning as to why it shouldn't be in the base game, nor do I see that one in particular being 'cheaty'. They already have it in the game in a sense with how policy/voting works; plus the practically exact same 'cheaty' feature they already added in for recalling companions to your party. Don't add on the excuse of 'full conversations' too about the 'unrealism'; we know there's practically no content in terms of dialogue with lords anyways even with a messenger system which doesn't affect much.
Sure it can be subjective if you don't want the messenger system in the game, I'd assume you would also have to advocate removing insta voting/policies, 'messengering' recalling companions, remote workshop changing, insta ransom notices, insta marriage offers, insta quest completion/awards (certain), etc...those follow that same 'messenger' aspect that make the game 'cheaty'.
I see those all as QoL aspects. Yes, not all players and their own requests/ideas should be implemented in game, but that is a bs excuse to use for not adding things that should be in a game more than it shouldn't be. Catering to the minority, that's how you get the barest min. viable product (which BL is) because it's the lazy thought of 'Oh, 5% of the playerbase doesn't prefer X feature despite the other 95% playerbase wanting it; best not to add it in then!'.