WoW addons are LUA files using Blizzard’s API built in Blizzard’s sandbox that Blizzard created for addon makers. It is not stand alone software and not really third party. It can only change the user interface display of data that Blizzard provides - within the rules that Blizzard sets. Blizz breaks those if they don’t like an addon. People have gotten banned for it, but only when they really misuse one in seriously detrimental ways that Blizz never imagined. Essentially exploiting the sandbox API to do things that are not so good.
Third party software, as in stand alone software and overlays are very much against Blizzard’s rules. You get caught with an overlay that touches the game and changes anything about it, you can be penalized.
Avalon already linked it, but yes, they are very clear about not using third party software with Diablo games (or any other game).
They don’t get funding from Blizzard… they get funding from advertising. Blizzard is supportive of them, but not financially.
Oh yes it does and you gave it permission to do so. It works like a virus scanner and looks for specific signatures of known cheating software. If found, it reports back.
Excellent advice. Just use a second monitor or device for any guides, websites, or other resources you want to have open. That keeps it separate from the game and keeps your account from risking anything.