Basically the title

Unfortunately any possible way to handle this on servers leads to broken vanilla inventory management. So I can't really implement it.

I'll leave this here instead of making a new issue since it's kinda related.

locked empty slots can still pick up dropped items.

Well... that one seems even harder to fix. :)

Yes! this is rather annoying and for a simple shift click action the complexity of fixing it considerably higher the one might expect.

It requires reimplementing most of the inventory handing in the game. I'll continue looking into it but I doubt it will be a fast fix.