Describe the issue

We need to clarify in the Contribution guide which ECMA version and target platforms should be supported, otherwise, we are doomed to have a lot of rudimentary code and inconsistencies.
Axios is written in a mix of ES3 and ES5 ECMA versions and it currently claims support for IE11 and newer platforms, but recent PRs use ES5+ features that are not supported by IE without any built-in polyfills. For example, we have trim util that has been deprecated since IE10, while we have Array.prototype.includes that IE doesn't support.

Example Code

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Expected behavior, if applicable

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Environment

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Additional context/Screenshots

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