In older versions of ActiveAdmin, it was possible to customize the admin footer using config.footer or even with a footer do ... end block inside resource/page registration. However, in ActiveAdmin 4.x, these methods have been removed or deprecated.
I initially tried using:
config.footer = "..."
and
footer do ... end
But both resulted in errors or had no effect in ActiveAdmin 4.x.

Solution:
Override the Footer Partial in Your App
Create the file:
app/views/active_admin/_site_footer.html.erb
write your footer as you like. e.g

<footer id="footer" class="footer flex justify-center items-center py-4">
  <span class="text-gray-500 text-sm">
    reach me here: ...
  </span>
</footer>