You can't just ensure a config change won't break things in large distributed systems, it's too complex with too many factors, there will always be risk. To mitigate your risk, youd want to design your system to do progressive, regional rollouts, with canaries to attempt to detect and isolate before a wide spread outage occurs. Even if you have all of this set up, there is still risk that your regions and systems are not fully isolated and outages could cascade anyway.
There will always be risk, there will always be errors. This is why SLAs and SLOs exist, they define and codify an agreement of what an outage is and what compensation is required if the agreement isn't met.
You can read Fastlys SLA here: https://docs.fastly.com/products/service-availability-sla