It is our understanding that there's kind of an unspoken agreement between the game copyright owners (Hasbro, who purchased Avalon Hill) and website developers that they are unlikely to interfere so long as you're not using any of their art or instruction text directly and so long as you're clear that your version is "inspired" by the original and provide attribution. This is the reason that every site you visit has their own handwritten version of the rules, for example.

In our case, we really do encourage people to go buy a copy of the board game, preferably from your local nerd shop. There's a lot to recommend playing Diplomacy online (I believe it was the second game playable by mail published after chess), but there is something special about playing in person and having all the physical materials.

It's not really so much of an unspoken agreement as the fact that they can't do jack squat about it. Game mechanics can't be copyrighted because they're not a concrete form of expression, only the art and instructions can be.

It's legal so long as you change the name, and don't use the same graphics.

I made a website for a fork of UNO years ago called "Hot Death UNO" and was sent a cease and desist from Mattel for it. I changed the name and the look of the cards and poof, no more problem.

That's a question I'm clearly interested in! I just moved to another country and am working on a website to play my favourite board games with my family and was wondering if I could publish it on github or if I should keep this private as I don't have the rights for the board game.