There's a new mandatory - 5.3.0.0. It corrects a bug in the new solo cruncher fix that made it so that not all wallets correctly used that fix for all accounts. If you don't upgrade, you can fork at any point in the future if one of those users stakes. There is no hardfork block, so you will need to upgrade immediately because the unfortunate nature of the bug.
For more information, see the Reddit post about the release