So, you can still update your game and your itch page for 24 hours after the deadline. I think this window is mainly meant for last minute bug fixes and polishing your game page. Since the rating phase starts right away (as far as I know), it gives you a chance to quickly fix any issues that people point out. You can technically add more features, but I'm pretty sure it goes against the spirit of the grace period.
The one think I know for sure is that if you submit early, you can still update your page and change your game files. I was afraid that once I submitted my project that my files would be locked with whatever I had at the moment, so I put out a post asking about it and I the answer I got was that you are allowed to keep updating your game after submission. Personally, I think it might be best to submit your project once you have something you are happy with and then update from there, just in case itch has issues near the deadline. I've heard stories of big jams (like GMTK) causing itch to slow down or crash, and people who waited until the last minute couldn't submit. I think they gave a grace period for submissions too, but i'm not totally sure.
Please take all my advice with a grain of salt. This is my first jam, and this is just my understanding on the problem, and I could be completely wrong.
Hope this helps!
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Uh, I submitted to make sure I was in but I don't see a way to do so. It says there are no projects on your account you can submit to this Jam... Did I just lock myself at a Beta release and miss out on tomorrow?
"There are no projects on your account that you can submit to this jam. Please create a new project or publish an existing one from the links below."