I don't think it should be illegal, for two main reasons:
1. It's not enforceable. Maybe it is actually, I don't know how exactly the ignore list works. But it seems at least like it would be hard to enforce. Even if ignore lists are illegal, it's impossible to enforce manually ignoring players by just not looking at their posts; ignore lists are just an automated way of doing that.
2. I don't think it provides anyone with an advantage to ignore other players, at least not an advantage that can't already be gotten by other means. You can already search for posts by a player, in some sense ignoring some set of players is the same as searching for posts by the complement of that set. In theory the only advantage you get from ignoring players is that you have less to read, but you can already do that by just skipping over those players' posts. Giving yourself less information is strictly bad for your alignment.