Nobody writes code that fails reliably under expected conditions. Reliability is a question of robustness in the face of unexpected or rare conditions.

BTW, chat apps are a bad example. Not sure why this thread is using them. WhatsApp has higher uptime than the telephone system itself, if I recall correctly. Making highly scalable and available message switches is a solved software/computer engineering problem.

A better example is most bespoke business software. If it collapses in a messy heap because a microservice ran out of disk space, people shrug and put in alerts to make sure disk space doesn't run low again without an operator being notified. Job done. Not exactly a rigorous approach to failure but it'd make no sense to spend weeks or months on making the entire system fail gracefully in that situation when it's easily avoided.