Access to core comment preprocess variables in a custom comment layout twig template? In Drupal 8 I have a custom comment type and when I use the default core comment.html.twig to render the comments then variables like 'new_indicator_timestamp' and 'parent' get rendered as expected. But if I switch to using a custom layout (via layout_plugin and display suite) and copy across the relevant twig template code for those core comment preprocess variables into my own custom layout then they don't work.

What do I need to do to ensure that the comment.module template_preprocess_comment() variables are available to my custom comment layout?
Probably the twig changed, layout modules usually bring their own templates. You have to check which templates are used. To find this out use twig debug:

https://www.drupal.org/node/1903374

After you found the right twig you have to use the corresponding preprocess hook, which you find in the comments of the twig template.