This is out of the question for any normal user and is also not reliable at all. When you install a Jailbreak you are by default giving up on a plethora of apps which do proper checks and they won't work at all even if you try to bypass them (banking especially).

Older iOS versions have other problems as well. Safari is not an app that you can update, it's part of the OS. A lot of websites will simply stop working for you if your version is too old.