BFGs can still be traded, but the team (BFG) team that holds them will slowly move them out of circulation by sending them to burner addresses (i.e. burning circulation to reduce the supply)
There are also people who do the same as well involuntarily too. It is not talked about a lot in crypto world because that data is not proven to be calculated correctly. We have no idea who did what and that is why we do not know how much is truly "gone". This applies to ALL coins and tokens as well. Just for example, bitcoin has about.. what now 18 million? And how much of it is in actual play? Daily if you ask me 3-5 million is traded between people at even the most volatile days and that is why I believe that we should not be really focusing on what the team is burning only but realizing that most of the money here will be gone in the future as well.