Mitnick's hack probably wouldn't work today but I don't know the specifics of who he was listening into nor with which gear he was using. Most P25 networks these days are set up with trunking, so the conversations hop around a bunch of frequencies at random. Holding down the transmit button would do nothing to interfere with the conversation taking place. Even with sophisticated gear you wouldn't be able to who was transmitting at any given time so you'd have to be willing to jam all the frequencies in a group.

Of course, P25 systems are still sometimes set up without trunking so in some situations it might work.