Apple Explains why iOS 18.7.3 is only available to some users still on iOS 18

So the next question that a user might ask would be what iPhones will iOS 18.7.3 support?

That would be iPhones that do support iOS 18, but do not support iOS 26.

It's a limited list.......the iPhone Xs, iPhone Xs Max and iPhone Xr.

Very sad decision by Apple to hold iOS 18.7 users hostage to try to force adoption of the problematic iOS 26, watchOS 26, etc. If the security issues are acute as they say, iOS 18.7.3 should be available to all iOS 18.7.x users. I’ll take my chances and continue to avoid iOS 26.

Did you read the referenced article above? Obviously the Apple tech support person did not. It states for all iOS 18.7.3 security updates they are...

Available for: iPhone XS and later

That doesn't imply that this update won't be available for newer iPhones; in fact, just the opposite.