I just added a trade deadline to Basketball GM and Football GM. By default, it happens 60% of the way through the season, but you can change the timing or completely disable it in the God Mode settings under "Trade Deadline".
The trade dealine is visible in the list of games at the top of the screen and on the schedule page. There is also a new entry in the Play menu, "Play until trade deadline". These will only appear for new seasons started after loading the latest version of the game. So for existing leagues, you will have to wait until next season to have a trade deadline.
For a long time I resisted adding a trade deadline, because I always thought the trade deadline was kind of stupid. Even 60% of the way through the season, you pretty much know who the contenders are. So what's really gained by not allowing trades after then? There are edge cases, like when a major injury late in the season ruins a contender. But what do we gain by forcing them to keep their other players?
Then again, part of making a sports simulation game is realism, and it just feels weird to see team making major trades right before the playoffs. And even if my personal preference is that the trade deadline doesn't matter much to me, I know a lot of you guys disagree with me, and your opinions do matter to me!