Is the used vulnerability closed after a jailbreak?
Jailbreaking doesn't immediately make older iOS immune to vulnerabilities with newer iOS, but it has been possible to install tweaks which protect against certain exploits.

These might not be the same implementations as with the newer iOS, but serve to protect against specific exploits. For example, SpectrePatch is a jailbreak tweak for older versions of iOS to protect against Spectre.

A survey of a broad range of security professionals would be iOS devices are less secure when jailbroken. 


https://www.tomsguide.com/us/iphone-jailbreak-risks,news-18850.html