That’s a nice turn of phrase:
Forever day is a play on “zero day,” a phrase used to classify vulnerabilities that come under attack before the responsible manufacturer has issued a patch. Also called iDays, or “infinite days” by some researchers, forever days refer to bugs that never get fixed—even when they’re acknowledged by the company that developed the software. In some cases, rather than issuing a patch that plugs the hole, the software maker simply adds advice to user manuals showing how to work around the threat.
The article is about bugs in industrial control systems, many of which don’t have a patching mechanism.