Security through obsolescence is the use of obsolete technologies whose vulnerabilities are no longer well known among the public.
Less common or obsolete software has fewer issues with malware as it lacks the market share that would make it attractive to a hacker. Furthermore, obsolete systems can be difficult for an attacker to target as the design, flaws, protocols and even programming may have fallen out of common knowledge.
Although security through obscurity is generally deprecated, security through obsolescence is still sometimes used in networking, such as the use of antiquated X.25 networks by ATMs.
Security through obsolesce, like security through minority is a strategy best confined to closed source software. While there are rare and obsolete variants of open source software, their source code is publicly available so that it is much easier for an attacker to find vulnerabilities.
Obsolete but still heavily used software can be about the worst possible option, however. Software at the end of a long lifespan, such as Windows XP, can provide a large target base. There is likely to be existing malware targeting it, while patches and support may have been discontinued.