The support for ES5 only browsers has been dropped with T178356: Raise Grade A JavaScript requirement from ES5 (2009) to ES6 (2015) we can upgrade the code used in our projects to ES6,

This can mainly be done by bumping the eslint rule set applied on the source from wikimedia/client-es5 to wikimedia/client-es6 in the .eslintrc.json file. Then the linters can run with autofixes.
It's also advisable to bump the core requirement in the extensions.json to at least "MediaWiki": ">= 1.41.0" if the code changes are user facing.