How to add a “Last Modified at” field in footer area?
You can add a View in views, create a block display, give it a contextual filter of node id from current URL, then output the Changed field and select a date format. You can change the label to "Last modified on" and you're done. Then just place that block in the right region.

Sorry I don't have code or pics, posted from mobile.

Edit:

You can add a last modified date in the footer area of page.html.twig:

{% if node %}
  &lt;p&gt;Last modified at {{ node.changed.value|format_date('custom', 'm/d/Y') }}&lt;/p&gt;
{% endif %}


This will work also, first is a no-code solution. Example, perhaps your requirement changes and you need to also show the time next to the date. That is just a simple update to the View, no twig edit + git + deploy + etc needed. Some prefer one way over the other.

Also, and I am going out on a limb here - you should know that node.changed only contains the last modified time on that node. If you changed other things on the page, like menu links in the header, or added new blocks, that date value won't change, because they are not part of the node object being viewed.