JavaScript failing on older iPhones – iOS 13

It looks like the Commercial JavaScript bundle is failing to run with a Syntax Error on iOS 13. This is causing crosswords to fail, and probably other things. I'm not sure if this is a bundling issue on the WebX side or a syntax problem within the commercial bundle

Originally reported on crosswords. See conversation:
https://groups.google.com/a/guardian.co.uk/g/liveblogging.dev/c/2XtmcuOVBdI

Start by reproducing the Syntax Error on an iOS 13 device or simulator, then inspect the Commercial JavaScript bundle and the WebX bundling path mentioned in the issue. Done means the bundle runs on iOS 13 and crosswords work again, with the cause identified as either bundling or unsupported syntax.

Written by the indexing model from the issue text.