Rules for nullish coalescing and optional chaining

I noticed that babel-preset-airbnb added the following stage 4 proposals in [v4.2.0](https://github.com/airbnb/babel-preset-airbnb/blob/master/CHANGELOG.md#420---20191114):

- @babel/plugin-proposal-nullish-coalescing-operator
- @babel/plugin-proposal-optional-chaining

Are there any rules/guidelines for these features?

Start by reading the linked babel-preset-airbnb v4.2.0 changelog entry and the issue’s references to the nullish-coalescing and optional-chaining Babel plugins. Determine whether the project has an agreed position on rules for these features; done means the guidance is decided and documented in the style guide.

Written by the indexing model from the issue text.