My only long-standing subscription right now is The Writer.
All the issues I've downloaded (going back to July) are still on & readable by my N2E.
My Nook library online will only show the most rescent 4.
Other inactive subscriptions have let me retain the issues, no matter whether archived ot not.
When I'm near Wi-Fi again, I'll try archiving some of those older Writer issues and see if they show up in my library online & provide an update.
I've read, but not verified myself, that if you archive the issues and then unarchive them you'll always have access to them.
Page 108 of the manual, "Automatic Deletion of Periodical Issues", explains what the various retention policies are.
As always, I recommend making backups of any digital products that you want to keep. Don't rely on B&N or anybody else to keep them available, and don't rely on your NOOK (which can go from "fine" to "irreparable" in the time it takes to fall a few feet to the floor).