The Firefly CCB discussed this issue today.

As an information item, Safari 9 was introduced with OS X 10.9 (Mavericks). Apple ceased providing update to it in 2016. All later versions of macOS (10.10 through the current 10.14) support at least Safari 10.

LSST has no objection to this change.

Before:

Using plugins:
check-es2015-constants {"safari":"9"}
transform-es2015-arrow-functions {"safari":"9"}
transform-es2015-block-scoped-functions {"safari":"9"}
transform-es2015-block-scoping {"safari":"9"}
transform-es2015-classes {"safari":"9"}
transform-es2015-destructuring {"edge":"14","safari":"9"}
transform-es2015-for-of {"edge":"14","safari":"9"}
transform-es2015-function-name {"edge":"14","safari":"9"}
transform-es2015-object-super {"safari":"9"}
transform-es2015-parameters {"safari":"9"}
transform-es2015-spread {"safari":"9"}
transform-es2015-sticky-regex {"safari":"9"}
transform-es2015-unicode-regex {"safari":"9"}
transform-regenerator {"safari":"9"}
transform-exponentiation-operator {"safari":"9"}
transform-async-to-generator {"edge":"14","safari":"9"}
syntax-trailing-function-commas {"safari":"9"}

Using polyfills:
es6.typed.array-buffer {"safari":"9"}
es6.typed.int8-array {"safari":"9"}
es6.typed.uint8-array {"safari":"9"}
es6.typed.uint8-clamped-array {"safari":"9"}
es6.typed.int16-array {"safari":"9"}
es6.typed.uint16-array {"safari":"9"}
es6.typed.int32-array {"safari":"9"}
es6.typed.uint32-array {"safari":"9"}
es6.typed.float32-array {"safari":"9"}
es6.typed.float64-array {"safari":"9"}
es6.map {"edge":"14","safari":"9"}
es6.set {"edge":"14","safari":"9"}
es6.weak-map {"edge":"14"}
es6.weak-set {"edge":"14"}
es6.reflect.apply {"safari":"9"}
es6.reflect.construct {"safari":"9"}
es6.reflect.define-property {"safari":"9"}
es6.reflect.delete-property {"safari":"9"}
es6.reflect.get {"safari":"9"}
es6.reflect.get-own-property-descriptor {"safari":"9"}
es6.reflect.get-prototype-of {"safari":"9"}
es6.reflect.has {"safari":"9"}
es6.reflect.is-extensible {"safari":"9"}
es6.reflect.own-keys {"safari":"9"}
es6.reflect.prevent-extensions {"safari":"9"}
es6.reflect.set {"safari":"9"}
es6.reflect.set-prototype-of {"safari":"9"}
es6.promise {"safari":"9"}
es6.symbol {"edge":"14","safari":"9"}
es6.function.name {"edge":"14","safari":"9"}
es6.regexp.flags {"edge":"14"}
es6.regexp.match {"edge":"14","safari":"9"}
es6.regexp.replace {"edge":"14","safari":"9"}
es6.regexp.split {"edge":"14","safari":"9"}
es6.regexp.search {"edge":"14","safari":"9"}
es6.array.from {"edge":"14","safari":"9"}
es7.array.includes {"safari":"9"}
es7.object.values {"safari":"9"}
es7.object.entries {"safari":"9"}
es7.object.get-own-property-descriptors {"edge":"14","safari":"9"}
es7.string.pad-start {"edge":"14","safari":"9"}
es7.string.pad-end {"edge":"14","safari":"9"}
web.timers {"chrome":"62","edge":"14","firefox":"56","safari":"9"}
web.immediate {"chrome":"62","edge":"14","firefox":"56","safari":"9"}
web.dom.iterable {"chrome":"62","edge":"14","firefox":"56","safari":"9"}

After:

Using plugins:
transform-es2015-destructuring {"edge":"16"}
transform-es2015-function-name {"edge":"16"}
transform-exponentiation-operator {"safari":"10"}
transform-async-to-generator {"safari":"10"}

Using polyfills:
es6.symbol {"edge":"16"}
es6.function.name {"edge":"16"}
es6.regexp.flags {"edge":"16"}
es6.regexp.match {"edge":"16"}
es6.regexp.replace {"edge":"16"}
es6.regexp.split {"edge":"16"}
es6.regexp.search {"edge":"16"}
es7.object.values {"safari":"10"}
es7.object.entries {"safari":"10"}
es7.object.get-own-property-descriptors {"safari":"10"}
web.timers {"edge":"16","safari":"10"}
web.immediate {"edge":"16","safari":"10"}
web.dom.iterable {"edge":"16","safari":"10"}

Shall we update the README file in Github to list out the supported browsers and versions?

You right, I will do that as part of the ticket.

The scope changed on this ticket we needs to update webpack as well, added two story points.