Your saves from v1.3 are NOT compatible. Your hats and settings are fine, though.
Multiplayer is now playable in Box Party!
Big thanks to emmyposs on Github for helping squash out a bunch of multiplayer related bugs.
Look up guides for setting up multiplayer in GZDOOM if you're interested in trying it out yourself.
New Features:
-New burn status effect: Doesn't stun enemies, but deals damage over time. Has synergy with shock status effect.
-All pepper enemy attacks can inflict burn effect on other enemies. (Except to enemies who are immune.)
-Added 8 new secret hats. There are now 30 hats accessible in Box Party Demo.
Fixes/Changes:
-HUD shaking no longer causes desyncs in multiplayer. (thanks emmyposs!)
-Side Quest UI now works and updates properly in multiplayer. (thanks emmyposs!)
-Opening the hat menu no longer causes desyncs in multiplayer. (thanks emmyposs!)
-Players now share keys in multiplayer.
-Added more quit messages.
-Improved compatibility with Xaser's DamNums mod.
I will be implementing more QOL changes for multiplayer in the near future.