Great question — answering as the UphillTech team:
Adding a custom field to conditionally display last modified date: You can use the built-in custom fields metabox of your post. Usually it is hidden by default, so you have to enable it going to 'Screen Options ' and checking the 'Custom Fields' box like this: If you're using the new block editor, you'll find the option by going to the Edit Post Screen > Click the three dots button on the top right corner > Settings > Advanced Panels . Now, add a new custom field, let's call it show_last_modified and set it to true. Displaying the last modified date: The function the_modified_date() will give you the date on which the post was last modified. To display it, seems that you will have to...
Full answer by Himad: https://wordpress.stackexchange.com/a/363745 (CC BY-SA 4.0).
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