Everything in the project will be packaged.
So, the way around that is to make a new, empty project, and migrate your main map(s) to that project. The migration will take only that has been used.
Then package from the migrated project.
I don’t think it’s possible. Because you could be dynamically referring to assets during runtime from blueprint or C++, and they have no way of knowing which ones. I guess they could have a tickbox ‘no dynamic references’, but then that’s adding to the complexity of the packaging…
Hi,
I am new to Unreal as you can understand. I wonder if I import some assets from the marketplace (for example car materials from epic) it downloads and adds a ton of assets, materials etc to my project.
I wonder if this slows my build time or will eventually added to my packaged project or will they be detected and ignored by the compiler?
Hmm I see. But I would like Epic to develop a feature like clean up. That would be nice.
In this case migration should have the same problems?
Yes, actually 
I guess if you’re dynamically referencing stuff, it’s your own problem…