It could mean one of three things.
1) The most boring - Sting just doesn't work for TNA anymore, but he's going nowhere special. This is what it is most likely to be.
2) Sting has signed for the WWE. This is what the vast majority of fans want to see, I'm sure, and TNA have taken him off because he is no longer with them. The best evidence for this is that he didn't have a leaving storyline, and he's gone via a minor website alteration, not some far reaching angle.
3) This is the TNA wildcard option. TNA knows that there's a shit load of talk about Sting going to the WWE. However, if he had in fact, as was initially reported, re-signed with TNA, then they would be able to play this ace. Simply put, if Sting shows up on the next TNA PPV, it would be an incredible surprise, and TNA would have acheived it without signing anyone or without even investing much into it. It would be a smart move, which is why I doubt they've done it.