Custom layouts for D8/9

Problem

I followed the advice here (https://www.drupal.org/project/radix_layouts/issues/2437653) placing the custom layouts in my sub-theme and did a cache clear, but received an error: Drupal\Component\Plugin\Exception\PluginNotFoundException:...
I am using D9. What about the radix_layouts.layouts.yml file, does that need to be updated with the new layouts? Should that file exist in my subtheme/panels/layouts?

thanks