You are misreading the release notes.
What was fixed is a Redex model (a tool used in semantics to model computer languages). A change to the modeling of `set!` (assignment) was made. It has no impact on Racket itself.
Unless you're the one who did the work, I'd rather trust the release notes than your unsourced assertion.
That said, just because a bug was fixed in an important function shouldn't be an indictment against the language. Racket is stable and productive, and formal modeling can reveal all kinds of corner cases which may not have been exposed in typical use, and may not have security implications. Maybe it was edge-case performance, maybe garbage collection, the release notes simply don't say.
I'd rather use a language whose authors improve their tools to discover and fix issues than one where this doesn't happen.