See:

• Make format control restrictions experimental #74739
• Restrict formatting controls in write mode (contentOnly) #71058

#71058 introduced the new private API to restrict inline fomatting controls, and only format controls marked as "essential" are availavle when in contentOnly.

Since the essentialFormatKey is a private Symbol, so there is no way for consumers to mark their formats as "essential". This behavior has also been reported as a bug:

• Inline Format missing in Synced Pattern with Paragraph and heading Overrides (WP 6.9) #74168
• Inline-Format Toolbar is missing for custom defaultBlock's RichText components #74345

Let's clarify the expected behavior of this API more, explore problems to be fixed, and aim to stabilize this API for WP 7.0.

Based on feedback, it seems like a good idea to remove this API.