Items in the backslots will still drop on the ground with this grave mod https://www.curseforge.com/minecraft/mc-mods/youre-in-grave-danger

At https://github.com/B1n-ry/Youre-in-grave-danger/blob/1.18.2/src/main/java/com/b1n_ry/yigd/mixin/LivingEntityMixin.java#L58 the items get added to the grave item list and since in the PlayerInventory class only those three "main, armor, offHand" are public instead of the "combinedInventory" (where BackSlot adds its items) which contains all three of these :/

My guess is that an AW onto the combinedInventory would work fine and then this can be used.

Added better compat with other grave mods. yigd with f3f3aef
It looks like the different grave mods just copied the same code xD
Except for https://www.curseforge.com/minecraft/mc-mods/universal-graves, this one works perfectly fine.
The compat I have with all other gravestone mods is not perfect since they don't use the combined inventory field (which is private, could be changed with an AW)

Bad compat:
https://www.curseforge.com/minecraft/mc-mods/youre-in-grave-danger
https://www.curseforge.com/minecraft/mc-mods/gravestones
https://www.curseforge.com/minecraft/mc-mods/charm (preserving totem)
Good compat:
https://www.curseforge.com/minecraft/mc-mods/universal-graves