Hi there. I think you might be posting about the wrong plugin. I just gave mine a test and it indeed works with WordPress 4.9.
Is this the plugin you’re referring to?

Yes, its the plugin.
I use WP 4.9. And my site are https://vedsegaard.dk/ as You can see, it dosent show up
anymore.
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This reply was modified 3 years, 2 months ago by
vedsegaard.
Hmmm. I’m not familiar with how your site normally looks so I can’t tell where it should be.
Can you show/tell me how you’re using the plugin to display the last modified time?
It should normaly be in the top, just under heading.
How are you adding the timestamp there though? What is the code you are using? Shortcode or PHP function? This plugin does not add it automatically anywhere on the front end.