Ubsan will abort an invalid program if it detect ub. It doesn't let you handle it. So you shouldn't remove the erroneous check, but fix it so it is no longer erroneous, and ubsan will help you identify these errors.

Also ubsan adds significant overhead so it is not really appropriate for production builds unfortunately (hence my wish for a less powerful ubsan-lite but with lower overhead).

    if (!p) {
        use(*p);
    }

What people are complaining about is the opposite case:

    use(*p);

    /* The compiler reasons that if p == NULL, the program would have crashed by now,
       so if we got here, p != NULL must hold. */

    if (!p) {  // the compiler can remove this branch
        report_error();
    }