Nothing’s a sure thing until it goes live.
That said, I did a whole lot of digging and found the very first mention of 5.0, by the devs, all the way back in August 2019.
Watch Developer Q & A (All Things 4.5, Deeds & Challenges) from infinityplustwo on www.twitch.tv(Link goes to the appropriate question; takes ~30-45 seconds to be answered)
It’s brought up during a question about banner mana combinations. At that point in time, Nimhain and Sirran don’t explictly state “third map”, but Sirrian super-heavily implies one when they say “a place alongside the Underworld” and “5.0” in the same sentence.
I’m reasonably sure that this was clarified a bit on a later stream somewhere (may not necessarily have been a Dev Q&A stream) to be a third map, of an unknown location (I remember some comment about the map change button being modified to select which map to go to when it does eventually occur). But, that clip took me over an hour (almost 2) to find and I’m calling it quits on this here.
To be 100% fair though, I did technically analyze this in the stream notes and decided in the moment that this was sufficient of a tease to be called a “third map”.
And apparently, through all of this digging, that I took up the mantle of writing stream notes from the previous person all the way back at 3.5, in mid-2018. I’ve been doing this for almost two years now. Gaard, I’m virtually old… or partially insane, or probably a bit of both. 