it has nothing to do with side loading.
it´s just an option who need to be turned on to develop app. When developers build app on iPhone, they access at some resources to mesure performances. Some people used that to bypass security.
so apple added a switch to force the user to turn this option on in order to activate these features.
I can’t use Google chrome beta and Google calendar without turning On Developer Mode On. Running TestFlight apps requires developer mode On fyi anyone.
Edit: it’s iOS 16 bug and will be fixed beta 2. So hold off anyone wants to install TestFlight apps or don’t update them. Turn off automatically in settings.