I am also looking for exactly this feature. The longer I use Evernote, the more I need this to continue to have it to be a productive tool for me. I have too much clutter in my notebooks but occasionally want the ability to search everything I have ever entered. This is even more important with paid accounts as its not as easy as just creating another account for archived information.
If I could make a note or entire stack as archived. When archived, the note does not appear in the left panel or any searches. There would be new options to "Show Archived" for the left panel or "archived:" in search to include archived notes. With this method, I don't need separate stacks, exports, or any of the other workarounds which really don't solve the problem.