Thats an interesting suggestion, it might make sense to tie this to the clutter setting so that they can only be recovered if the setting allows it, or sometimes requiring gluing, as they are a ruined vessel, an anchient artifact and so quite delicate. That would be the vanilla explaination of why they can only be broken. Though the ones you get from traders can be moved around quire readily, but not after placing which is somewhat of a contradiction,
As far as making their collection only possible after the steel age I can see gameplay reasons for this but I dont really see a method I could do to gate it that way, not one that makes sense anyway, perhaps some kind of temporal method could be used to allow collecting them, a jonas device or mechanical part.
but I just think if traders can mive these things around with ease why canyt players? Maybe those traders are just malefactors?

https://www.vintagestory.at/forums/topic/12774-cracked-vessels-should-be-pickable-in-some-way-in-the-lategame/

It's cool that you have done this awesome mode. I have a little sugestion about it. I think it wiil be better to make some tool (iron or even steel) for picking vessels so it will not be so easy. I defenetly will use your mod in my next surviwel world but limit myself to take vessels only after steel pickaxe