Ideally, before upgrading, we can get rid of all deprecated APIs surfaced by StrictMode of React 16.6.

• findDOMNode
• The old contextAPI
• componentWillReceiveProps and other lifecycle hooks.

Remaining instances (feel free to add items to the list when you notice used deprecated APIs)

• Remove findDOMNode from Tooltip Remove findDOMNode from Tooltip component #11169
• Remove findDOMNode from DropZone Provider Try avoiding the deprecated findDOMNode API from DropZone Provider #11168
• Remove findDOMNode from Block Leverage forwardRef to remove findDOMNode from the block component #11170
• Remove findDOMNode from the inserter Remove findDOMNode usage from the inserter #11363
• Remove findDOMNode from NavigableToolbar Remove findDOMNode usage from the NavigableToolbar component #11401
• Remove findDOMNode from withHoverAreas Remove findDomNode from withHoverAreas by making a render prop component #11407
• Remove old context API from Slot/Fill Update SlotFillProvider to the new context API #11123
• Remove componentWillMount from LatestPostsEdit Remove componentWillMount from LatestPostsEdit #14637
• Remove componentWillUpdate from the Fill component Refactor fill component to avoid relying on componentWillUpdate and use React Hooks instead #15541
• Remove componentWillReceiveProps from TinyMCE Rich Text: Update TinyMCE component to use componentDidUpdate #8148 Remove deprecated componentWillReceiveProps from TinyMCE component #11368
• Investigate and Remove componentWillReceiveProps from Resizable (it's React Native code)
• Remove componentWillReceiveProps from Input at ColorPicker component Remove componentWillReceiveProps from ColorPicker #11772 @nosolosw