I did my interview today around 4PM. I didn't notice the note on the door, but I think you could probably still try. There were only a two other people there at the same time. I was in and out in less than 15 minutes. If they would have been willing, they could have definitely squeezed a few walk-ins in during the same time. A few days ago I spoke with someone at the TSA Pre sign-up area upstairs and they did mention they weren't taking walk-ins anymore. But after my quick experience today, I think I would have considered trying to walk-in.

Agents/officers definitely didn't seem happy to be doing the whole interview thing though.