Can we be sure that the argument will never include spaces? Otherwise we need to quote it.

: "${jail_copy_resolv_conf:='no'}" and the following knobs?
Or are these defined somewhere else?

Can you test(1) the return of check_kern_features directly?

if check_kern_features inet; then
     ...
fi

Else, per test(1), you should check both integers (the return code $?, and 0), using -eq

Same here

Prefer read -r so backslashes do not act as escape characters (in this case, it's mostly to appease ShellCheck).

Same here (read -r)?

I just thought there is no need in providing default value, since I check it the following way

if [ "$jail_copy_resolv_conf" = "yes" ]; then

If the variable isn't defined, this will turn into [ "" = "yes"] and the conditional will not be executed.

OK, how about using checkyesno?

checkyesno prints a warning if the variable isn't defined. We don't want this, as these knobs are optional.