What happened to Heroes V on this site? It's not there anymore. I bought this a long time ago from the publisher and their site sucks so bad I cannot figure out where to download it again, so I figured I would buy a copy on GOG but alas it isn't here anymore. Any idea if it will be back?

Interesting. It looks like Assassin's Creed has also disappeared from the catalog, so I'm guessing Ubisoft decided to pull some of their more recent titles for some reason (the rest of the older Ubisoft games in the catalog still seem to be there). As to whether it will appear sometime again in the future, unless one of the GOG staff is willing to comment on the matter your guess is as good as mine.

It's still on my account page. You sure it isn't a site error?

Even if a game is removed from GOG's catalog it still remains available to download for those who have already purchased it (there have been one or two exceptions to this, but it was announced well ahead of time that this was going to happen). If you check the catalog, though, you'll see that HoMM5 no longer appears in it.

Still shows up for me.

Edit: and Assassin's Creed is still there as well, which I don't own.

It's showing up for me as well, while it definitely wasn't showing up before. Must have just been some kind of short-term glitch. So I guess the answer to the OP's question is "Now."