It is beyond silly to say that all my data is at risk because the OS I run isn't "supported" software. Older OS works fine. Next!
Well, now, don't be glib. There's SUPPORTED and then there's
supported. Perhaps you don't need SUPPORT by phone, genius bar, official forums, or feature improvements. However, just because a Mac keeps booting, don't assume you're out of the woods. An OS can be "broken" in ways most folk aren't ordinarily aware of. Those broken things are referred to as "zero-day exploits" when you have no advance warning, and as "CVEs" after a warning is published. For every CVE, there could be dozens more vulnerabilities yet undiscovered.
AppleTO is right, if a bit melodramatic. It's best to use a SUPPORTED version. However, Apple
supports OSs for security a while longer than they SUPPORT them in marketing channels. Regardless of one's "Ain't Broke" philosophy, it is plain irresponsible to blow off data and privacy security.
The moral of the story is: Don't be complacent. If you ever feel confident and relaxed about your data and internet experience, then you're not paying attention. If you need an
unsupported OS for critical apps, or just your comfort zone, understand that you should be compensating, manually, for a vendor's fall-off in security improvements. This can be a tedious, frustrating effort, indeed.