I don’t play AOE2 DE that much, and only actively play during season events (some rewards looks tempting to me). I do play a lot of Starcraft 2 so I’d like to speak from that perspective.
In Starcraft 2, all defensive buildings in the game has range indicator, yet the building is based on grid system, very similar to AOE2. I am not so sure how the mod is made or how it affects ladder gameplay, but just for the range indicator itself, it’s kinda big when playing certain cheese strategies (Protoss Cannon Rush for example). For Protoss Pylon, players often need to check Pylon range to make sure key buildings can be placed within that rage.
Since I am personally accustomed to this, I do not see why indicating tower range brings any disadvantage or advantage to the player. Quite frankly, it is nice to see where you want to build the tower for a bit of better coverage of enemy wood line or gold mine, which can theoretically help both Tower rushing or covering key areas of your own base by building towers at a more optimal spot.
It could be possible for Microsoft to review certain mods commonly used by community and determine which mod is allowed in ranked play and which one does not, and ban all mods that are not allowed during ranked play. Better yet, just like grid feature, bake it into a setting that player can enable, so the game is easier for players and potentially improve competitive play.
That is not to say that “Then you want to see the firing range of all combat units for more info on the field”. The thing is, units can move, and will generally attack whatever enemy that is in range. The game that does allow players to see unit attack range is Supreme Commander Forged Alliance, and I just keep that setting off because there are too many circles on the field. Range indicator works great for defensive buildings (Castles, Towers, Krepost, Harbour), not so much for units.
I hope Microsoft can bake this into the game as a setting that can be enabled, and I will be happy if this mod can be used for online play.