The "software safety check" wasn't an optional extra. It didn't
exist at all! The optional add on was for an "AOA DISAGREE" warning light, which would have indicated that the MCAS might be relying on faulty data (if the pilot was even aware of MCAS in the first place) but wouldn't have actually stopped it from doing so.
Now, the updated MCAS will only activate if both AOA sensors agree. Which seems like a fucking no brainer that should have been the case from the start, but... yeah.