We are committed to keeping the world of Sanctuary a safe and fair place for all players, and our Game Security team constantly monitors for cheats and unauthorized modifications. All Diablo IV players agree to the Blizzard EULA as a condition of playing the game. The Blizzard EULA explicitly prohi...
"With that in mind, we want to make it clear that TurboHUD4, like any game-modifying software, is prohibited for use with Diablo IV. Players who install this kind of software will put their accounts at risk for disciplinary action, which can include permanent suspension."
Overlay type of mods are always kind of critical because they simply work in a very similar way to map hacks and bots, using often the same type of functions. Even if it wouldn't offer anything offensive it would still likely be something their anticheat systems catch as a positive.
Diablo 2 successfully had both open and closed battle.net options for decades, and while technically map hacks were still not allowed, I’ve never met someone who was banned for having them, or any other hacks, on open battle.net. It was fine, and gives people who want to hack a place to do so without needing to bother other players who tried to play legit.
You can see videos of turbohud used with d3 on youtube. It adds, for example, item/skill texts rather than just an icon, more detailed dps meter, and a larger minimap as an overlay, etc. I don't know if any of it should be considered cheating.
Also, for what it's worth, the TH4 site says it disables itself when pvp marked.