To be fair, Blizzard themselves had stated a number of times that the Diablo 3 forums could be used to have Diablo 4 discussions.
For example:
Hey all,
We’d like to encourage you to have a focused place to discuss your excitement about Diablo IV. This is intended to be a dedicated thread for that purpose (though you’re welcome to make your own in General if you’d like). Be advised that we might consolidate threads if they’re similar in topic.
Not sure what we’re talking about? Check out all the announcement information here:
Taken from:
The same cannot be said for D2R, hence why most D2R threads are met with players instructing the posters to post in the appropriate thread sections. That said, I don’t really mind (or perhaps don’t even care about) the Diablo 2 Resurrected threads popping up in D3 forums as some of them end up being moved over to the proper threads anyway.