I personally think that the “Enemy Approaching” message could stay, though your point is understandable, and I do think that an “Enemy Radar" type of powerup is a clever idea
I feel like the message ideally works the best at the beginning of the game, when players are most likely to be using the most basic blocks on their bed, like wool, and are less likely to have harder blocks placed. Especially with the use of TNT or a blastball (fire charge), the bed can easily be made vulnerable, so the message makes sense early game.
I’d say that this message (especially when it’s early game) is one of the key things that separates BedWars from EggWars. From the looks of it, BedWars is somewhat centered around defense, certainly more than EggWars. Beds early on in the game should be protected from rushers, and I think a warning of an approaching enemy is just enough to give the team a better chance of defending their bed. As many might know, rushers are a common player type in EggWars, and I personally think that the gameplay of EggWars and BedWars should be kept separate as much as possible.
An idea is that maybe it could be made so that the message stops showing up after a certain point in the game;
The farther (later) you are in the game, the more likely that harder blocks will be used around beds, and the more likely that there will be better defenses on the island as well. This makes it harder for the beds to be destroyed by other players, which (destroying beds) should ideally become more and more of a common goal the longer the game lasts (especially considering the upcoming Deathmatch phase), at least in my opinion
I recently played some rounds of BedWars (Teams of 4) and I have not destroyed a single bed yet, xD. A major part of that is my playing style of course, but I feel like in my mind, I know that if I try to destroy a bed I probably won’t succeed at it. Sure, maybe I’m a little rusty and could do better, but we also need to consider the defenses that are in this game: turrets, totems, hard/blast-resistant blocks, etc. At this point in the game, the “Enemy Approaching" could be seen as an unnecessary layer of defense, almost too much defense. With all of the protection a bed can already have, I feel like players should be able to be sneaky when destroying a team’s bed
Also, something I’ve noticed happen in BedWars is that, sometimes, not many of the beds will be destroyed when the Deathmatch begins. That is the point of the Deathmatch of course, but I feel like bed destroying should be a little more encouraged/made slightly easier before the Deathmatch begins. I’d say that the “An enemy is approaching your island!” message is a factor that can discourage players from trying to destroy beds later in the game
Tl;dr- The message should still exist early on in the game but not later in the game due to the buildup of defenses on islands
Edit: I know that the TNT Rain phase can remove defenses (like turrets), but there are blocks that are blast resistant and a TNT isn’t always going to land where the bed is