This mod gives players various mobility-related debuffs when carrying too many items (configurable). Currently these debuffs are implemented:
The mod will also attempt to look into item containers (like shulkerboxes), count the items in there, and multiply their weight by a factor (configurable per item, 0.5 by default). This has been tested as working with: vanilla shulker boxes, IE crates, and Improved Backpacks. The issue is that different mods encode container contents into item nbt in different formats, which may not work with my method (my method is to look for something on the form [{...}, {...}, {...}] and then interpret that as a list of items). Let me know if this breaks anything!
Nested containers aren't counted (so if you put a backpack into a shulker box, the contents of the backpack won't be counted). But really, in most cases packmakers should prevent containers from being stacked.
This won't check modded inventory slots (e.g. Baubles).
