To be fair, there were multiple wrongs in that piece of code: avoiding typing with the forward goto cleanup pattern; not using braces; not using autoformatting that would have popped out that second goto statement; ignoring compiler warnings and IDE coloring of dead code or not having those warnings enabled in the first place.

C is hard enough as is to get right and every tool and development pattern that helps avoid common pitfalls is welcome.

I don't see this. The problem was a duplicate "goto fail" statement where the second one caused an incorrect return value to be returned. A duplicate defer statement could directly cause a double free. A duplicate "return err;" statement would have the same problem as the "goto fail" code. Potentially, a defer based solution could eliminate the variable for the return code, but this is not the only way to address this problem.

Is that true though?

Using defer, the code would be:

    if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0)
        return err;
        return err;