Would acid reflex be deadly if left untreated?

Possibly: Long-term complications of uncontrolled reflux are esophageal strictures (that may need repeated endoscopic procedures to fix), Barrett's esophagus (a precancerous condition that will need regular endoscopic monitoring) and of course esophageal cancer. A lot of this will depend on some other risk factors such as smoking history, family history to name a few.