It was not in something you would call an “Advertisement”. It was not in commercials, print ads, etc. It was not on the Blizzard D2R purchase page or advertised features.
It was in the Press Q&A panel at Blizzcon 2021. That was a huge and long interview. They were heavy on "bringing it to modern/live Bnet which takes changes, that mods would not work the same way, BUT they have one sentence saying TCP/IP would still exist if it was your thing.
That said, nearly everything they published or said has huge GAME MAY CHANGE BEFORE RELEASE warnings on it.
Here is the relevant interview transcript.
https://diablo.blizzplanet.com/blog/comments/blizzconline-2021-diablo-ii-resurrected-press-qa
These two questions and the answers seem to have most of the info related to the topics at hand.
ComicBook (Logan Moore): One thing that’s been big with Diablo II’s history in a lot of Blizzard games in general has been mods. Can you guys talk about how mod support is going to work here?
Gallerani: Awesome. So, first off, you’re absolutely right. Like one of the reasons that the game is still alive and kicking and relevant for 20 years is the mod community; and to be clear, the mod community that exists around Diablo II is still there. That whole ecosystem is untouched. That will keep going. When it comes to mods for Diablo II: Resurrected, it’s going to be a little bit of a different game to mod.
First off, mods that actually hack the game, that inject things into the DLL… those aren’t really going to be as welcomed anymore.
I mean, don’t get me wrong… you guys will figure out a way to do it. I’m not going to tell you that it can’t be done,
but with the shift over to modern Battle.net, and us trying to increase security and preventing item duping and bots and other things like that, those types of mods aren’t going to be as easy to do.
However, I have to give mad props to Andre and a lot of our other engineers. We’ve taken a lot of the aspect of the game that used to be hard-coded and we’ve moved them over to Data; and so things that you would use to have to hack the game to do, you don’t know because it’s in Data. We’ve also done some cleanup.
It’s gonna be a little bit easier to read. So, yes, we totally encourage mods and everything like that for Diablo II: Resurrected; but it’s not to say that: “Oh, that mod that I really love for Diablo II is just suddenly going to work for Diablo II: Resurrected.”
DualShockers (Ricky Frech): So kind of on the same topic of mods. Is there going to be any offline play for this new release, or is it all online?
Gallerani: The short answer is yes. All the ways that you used to be able to play the game are going to be the same. We’ve cleaned up the verbiage a little bit.
So right now you either make an online character cause it’s saved on Battle.net, or you make an offline character because it’s saved locally. So you have a local character, you have a Battle.net character. When you go online, you can go online on a game that’s totally private and you don’t invite anybody to it — we’ve made it easier to invite people into your game.
So you’ll have a friends list now. You can just click on your friends list and invite them straight to your game rather than calling them so long as your phone line, isn’t tied up with you being on the internet and telling them the game, name, and password.
You can just pull them right in, and then we now have a game finder. So you can just go on to the lobby and find other games, but you can also play it offline.
You can also do TCP IP connect if that’s kind of still your thing. So pretty much all the ways you used to be able to play, you can play now.
They’ll probably be some more modern, like verifying that the copy of the game you have is still legit, but that’s not going to prevent you from playing offline. That’ll be like, okay to start a game. Yeah. It’s legit. Maybe some period of time it’ll ask you again, but that’s kind of normal with not just most Blizzard games, all games nowadays.
The interview is huge so worth a read.