admin_post_toggle_custom_fields

See hook in action

See this hook used in plugins:

Select Plugingutenberg.4.3.0gutenberg.4.4.0gutenberg.4.5.0gutenberg.4.5.1gutenberg.4.6.0gutenberg.4.6.1gutenberg.4.7.0gutenberg.4.7.1gutenberg.4.8.0gutenberg.4.9.0