Online stuff lingers on in Roll20, but at the end of a campaign I make my players GMs in the campaign so they can peek at all the notes behind the curtain if they want.

I've got a spot on my bookshelf for all my old character sheets and notes from various campaigns. The last campaign I ran with physical notes has a big accordion folder for everything. I used it during the campaign so everything is still organized.

For me, mithral is the big one. The dwarves and dark elves are always fighting over it.