A long time ago I played Minecraft with this mod because it's my favorite, back then I simply equipped the backpack on my back but it didn't take up the back slot, I used elytras in the back slot, chestplate and backpack at the same time, however today I played with the mod again and I can't do it anymore, it takes up the back slot which reduces my utility space, is there any way to solve it?

Of course, you can disable this by changing "backSlotIntegration" option in server config to false

It worked, thanks for the help