Since "rare" and "occasionally" are not mutually exclusive (and neither can be pinned down to a specific quantity), you could say "Upon rare occasion" and I wouldn't quibble.
I'm sure we would also find agreement on the statement, "It could happen, but I wouldn't count on it."
To a large degree, this comes down to a basic truth; those seeking to find exploits tend to focus on the large, relatively under-exploited targets. For the most part an older OS ceases to be a target, and the newer features of a new OS receive the most scrutiny.
When Mac OS was around 2% of the installed desktop user base, it didn't get a whole lot of hacker/researcher attention. Now that it's closer to 10%, it gets much more attention. (And the much larger distribution of iOS has tended to make all Apple products a bigger target for researchers.) Once a new version of an OS has passed through a full year of active scrutiny, updates, and patches, for the most part the need to make further patches is diminished. However, when a serious exploit is encountered that affects multiple generations of the OS... security updates are still pushed out.