I've been a LastPass user a LONG time (850 entries plus 50 secure notes), and I'm also reluctant to throw anything away permanently. The result is that I have a ton of old "junk" in my vault, but there may be a gem in that huge pile. What I want is a way to remove the clutter, while keeping the option of looking through the pile, if I feel the need.
The idea is to split my vault contents into active and archive.
Those items I've archived would not show in "show matching sites" and would not show if I opened my vault and searched. But if I was spelunking a very old site, and figured I might have saved a password for it, I'd have the option of enabling view of the archives in both places.
As to how to get items into the archive state, it should be trivial to do so from the "show matched sites" drop-down, since that's is often cluttered with stuff that would take time to know for sure I could safely delete.
In order to do a mass vault split (into active and archive), the export function should have an additional column: a unique hash code that would uniquely identify each entry. Then I could use a spreadsheet to select a list of hash codes that I want placed into archive status. A new feature would accept a text file of those hash codes and set the archive status to true for each.