On the Vanilla side there is Shulker Boxes, while expensive/late game they are still a backpack, for modded there is many every version. Even in 1.14 I've come across about 3 already (and they are mostly new mods not ones that have updated that are well known which are more so on 1.12.2 I think). Take a look on curseforge and see what you find. Datapack wise however not too sure.
There are also Ender chests, which are an early-mid game item and can be combined with Shulker boxes for a ridiculous amount of storage (27 Shulker boxes * 27 slots * 64 items = 46,656 items; that's equivalent to 419,904 resources as mineral blocks, about the number I collect over 4 months of daily gameplay; as it is, an Ender chest alone is enough for 6-8 hours of nonstop caving). Not only that, you don't lose the contents on death and other players can't access it.
This appears to be a mod request, not a suggestion for vanilla Minecraft
As many people have pointed out, Shulker boxes fit this role in vanilla
The original post is not detailed enough to clearly indicate what exactly this mod request/suggestion would entail and how it would be different from existing backpack mods and/or methods to carry more items in vanilla (depending on if they intend it to be a mod request or a suggestion)
The OP is free to remake this thread, provided it is posted in the correct section and contains enough detail for others to go on.
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