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I always preferred CopyGirl's Wearable Backpacks, but there are many choices and that's a good thing.

They won’t be if they modify the registry after vanilla does

OR if you had some patience and don't need the fancy functionality from said mod (just the backpack system) you could just install Baubles and this mod who let you equip Shulker Boxes in the bauble's chestplate slot, and setting up a hotkey to open it like a backpack. For easy access to shulker boxes (by default you need to kill shulkers on the End) you can use CraftTweaker (or a datapack when 1.13 modding is out) to add shulker shells to a loot table like junk fishing, or create (with CT too) a recipe with accessible materials for this. But anyway just a random idea, imo more close to vanilla due to use of a existing block.

I live in a 3rd world country where 25 dollars is 1000 PHP to us and worth like my 3 months allowance, I pursued programming classes consuming my saturday weekends of time only to be uneducated by 3rd world quality programming teachers, the teacher would often just read the lesson in english which isnt our mother tongue language making it hard for majority of the students to understand, our lesson is takes 8 hours per saturday doing nothing,my classmates would usually sorround me in my computer to get help from me during the lessons, sometimes in each exam where we will need to build a program, my other colleagues would consume the 8 hour time and ask for more cus it aint enough, Me however who have experience in programming due to forge would just take 30 minutes to do it and leave my class and just save my allowance. I learned it all in programming with forge. So thank you forge, thank you forge staff, your sacrifices mean a lot to some people like me,

There was also the 1.2.x -> 1.3 transition when Mojang "merged" the client and the server code. There was no "common" code in 1.2.x, period. If you wanted your mod to work for multiplayer you had to write a separate version. I think there even was a separate API for multiplayer mods - Risugami's ModLoaderMP(although it might have became obsolete earlier)