Set default skin tone for all Emojis in Emoji keyboard in iOS 11? It appears that it is only possible to change each Emoji’s skin tone individually by tapping and holding on the Emoji key and selecting another skin tone in the pop-up menu.

Is it possible to set a default skin tone to be used across all the supported Emojis?
No. Due to the way it is implemented, it is not possible to set a default skin tone to be used across all the supported Emojis in iOS Emoji keyboard.

Each Emoji remembers its last used skin tone. Also, the recently used section shows all the skin tones variations used for a single Emoji.