Just received rapid security update! Is this legitimate

Yes, this is legitimate, see this exempt from the linked article

Rapid Security Responses are a new type of software release for iPhone, iPad, and Mac. They deliver important security improvements between software updates — for example, improvements to the Safari web browser, the WebKit framework stack, or other critical system libraries. They may also be used to mitigate some security issues more quickly, such as issues that might have been exploited or reported to exist "in the wild."

Timothyjebert wrote:

Just received an rapid security update, is this legitimate

Yes, as verified by the "Related Article."

Yes.

(239103)

Got a security request update. Is this real?