Would it be possible for a future update to prevent items from dropping to the ground when a player no longer has space available in their inventory? Instead of dropping the item, the inventory slot could be locked, preventing the player from placing anything else in that space. Additionally, a red warning message could appear, notifying the player that they have run out of space and need to remove items. This is just an idea, and I'm not sure how complicated it would be to implement. If it's something impossible, I apologize.

Locking individual slots is very tricky. I don't want to add that to this plugin unless absolutely necessary. I'll leave this open as a suggestion. If there's more demand, I'll consider ways we can address this problem (the "problem" being that items are dropped in front of the player when opening an overflowing backpack, if the player doesn't have enough space to carry all the overflow). There may be different designs that are more feasiable, such as a mail system where the player can receive the overflowing items (e.g., if there is a mail plugin, Backpacks could possibly integrate with it), possibly a command or container UI as part of the Backpacks plugin itself.

There is the ThePitereq Redeem Storage API and I don't know if I could do something with it.

https://codefling.com/plugins/redeem-storage-api