Hmm, but if people haven't finished the game yet, why would they even be on the Inquisition side of the keep? Why would they even be in the Keep while they're playing the game... they should be focused on finishing the game, not messing around with the keep. What good is it to you if you've already started the game? And how does the Keep have spoilers anyway, you have to actually click on one of the sections to have anything spoiled for you, and if you do and you spoil something for yourself, that's your own damn fault.
I'm in the party that the hidden is there to get people to replay the game.
I can see why they did this: they were concerned that people coming into DA:I would start filling out the Keep and just filling out tiles until it was finished, including the DA:I stuff. So they were basically worried about people not paying attention and accidentally spoiling the entirety of DA:I while setting up to play it for the first time.
What I take issue with is how they implemented it. This section could have easily been hidden behind a prompt that says, "This section contains spoilers for Dragon Age: Inquisition. Are you sure you wish to proceed?" Add another prompt that makes the user confirm that they really want to proceed. If they get spoiled at that point then it's on the end user, not them.
Honestly, we were all replaying the game anyway. I'm sure many of us have played the game multiple times by now. If their motivation was to get us to play the game again then they don't know their users. Those of us who are going to take the time to send in feedback on a regular basis are already doing so. The users who are going to continue to send in suggestions and report on bugs to help make the Keep better are already replaying the game and engaging on the forums. The people who are looking at the Inquisition tiles way before DA4 has even been announced are already their hardcore fans, so I don't see why they'd do something like this.