Good question. Hard to answer. However.......
We (DH, me and the 2 DC) moved from a large 4 bed house to a small 3 bed (1 room is a 6 X 8 boxroom,) as both DC were about to leave for University. We had about 65% of the room we had in the big house. So we decluttered. We got rid of about 50% of what was in the attic, and 30% of the stuff in the house.
Just over a decade later, I honestly couldn't tell you 90% of what we got rid of. (Couldn't have been that important, or valuable, or needed!)
Some stuff I will simply never get rid of. Parents and grandparents and great grandparents memoribilia, photos, and general stuff, things from my childhood, thing from my DC's childhood(s,) and 700 vinyl albums - going back as far as the 1960s, several dozen cassettes, 500+ vinyl singles, 2000 DVDs, and around 300 books.
No way will I get rid of the DVDs, as so much stuff is not only taken off streaming now, but also lots of stuff is whitewashed and sanitised, and edited within an inch of its life. So as to not upset the fragile and terminally offended. I'm SO glad I have all the originals of everything, to enjoy, and am not having my choices taken away from me.