Should I use the Junk Yard to store important notes long-term?
Short answer: No. The Junk Yard is for temporary scraps: every scrap clears on a timer (6 months at most), the oldest drops off when the board is full, and cleared scraps can't be recovered. Save important things elsewhere.
No — it's built for scraps you need for a while, not for keeping things permanently. Every scrap has a lifetime (up to 6 months at most), the board drops its oldest scrap when it's full, and a deleted or cleared scrap can't be brought back.
If something matters, save it somewhere proper — a document, a notes app, a folder. Use the Junk Yard for the in-between: the link you'll need this afternoon, a snippet for a reply, a screenshot you're about to send.