"no longer safe"
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Because you didn't get shiny new icons? What???
There are 0 unpatched userland-exclusive (you can run them in your web browser, without plugging the device in and running specific jailbreak programs) vulnerabilities for iOSes dating as far back as 4.2.1 --- and it's pretty safe to say that if one ever came out again, an update would patch it, even if a newer iOS came out. (see iOS 4.2 on iPhone 3G)
This has nothing to do with jailbreaking. Just as one example, iOS 6 and earlier and OS X earlier than 10.9 have font rendering vulnerabilities.