My understanding from post #81 is that people do get reminders.
Medical schools can see which students have certified their lists (I don't believe they can see the lists themselves, just whom has not certified)
GME offices can see which programs have and have not certified their lists.
My understanding is that, if you've certified a list and then edited it (and hence uncertified), the NRMP will allow you to reinstate your last certified list. Some fools inevitably try to change their list 5 minutes before the deadline, and have a computer/internet crash. I expect the same is true of programs.
There will be lots of people with never-certified lists -- those that are withdrawing for personal reasons, matched in another match, visa issues, etc. For the NRMP to reach out to all of them, with enough time to allow them to answer and still process the match, would likely require extension of the match window. Plus if people get a reminder email this week if their list is uncertified and ignore it, I doubt that an email on Thursday would be much more effective.
My bottom line: The NRMP should send some sort of alert email to everyone without a certified list 48 hours before the deadline.