It is already against the rules to post things not related to Diablo 3 (or 4 seeing as there is a sticky giving permission to talk about D4 here). The only thing they have not done is make a filter that removes things automatically, which I think is impossible and it would be easy to game by just putting the word Diablo in the topic.
The Diablo III forums are for discussion of topics directly related to Diablo III.
The forums here are specifically to discuss the game and related topics. Any topics not related to Diablo III, Battle. net, or Blizzard Entertainment are subject to deletion.
And this reminder about discussing post moderation of any sort (grey/deleted/modified)
Do not post about moderated threads.
Post that are moderated have a reason behind the moderation. It’s up to you to read the forum Code of Conduct and these guidelines to determine what you did wrong, learn from your mistake, and attempt to post again without breaking any rules.
References:
Sticky with Forum Guidelines: Welcome! Please Read
Forum Code of Conduct: Blizzard Support - Forum Code of Conduct
Sticky with permission to post D4 things here Diablo IV and Ongoing Communication