HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

We play a game on 3.45. When joining the game, this message pops up:
http://gyazo.com/dbb5e53ac7ae8b33bc84407d8f2c8cf4

About two weeks ago we played and everything was fine.

Well, this means the opponent either doesn't have HD+ or has updated their version.

Added after 13 minutes

Correct. Only it's been removed from the rules. And there's value, but no use. An inconsistency.

HD+ is created not only for HeroesWorld, many people play this version and therefore removing from the game what cannot be done on one resource but can be done on another seems wrong to me. Connecting a module like m200 is the most normal option. IMHO