Set up on Child Pages1 user found helpful
The online guide located here, suggests this is available, but to our knowledge it is not (yet perhaps):
You can, of course, make default content available to new pages. You will have to walk through the process to become comfortable with how it works. When adding default content to a page type (Go to Dashboard, select Pages & Themes, select Page Types, then click the Output button (for the desired Page Type), then click the Edit Defaults button), a new page will open in a separate tab, so it's a bit confusing at first. Turn on edit mode, make your changes, then save and return to the dashboard and return to your website and you should see your default content appear in the page the next time you select the page type for a new page.
Hope this helps!
Set up on Child Pages
This option allows us to "push" content from the default page type template defaults to every page on the site that currently uses that page type and template.
But I don't see this option anywhere.
You are correct, this feature in the documentation is not currently an available feature, unless its hidden somewhere and someone discerns how to get to it.
For the template you are using click on 'Output'
Click 'Edit defaults'
Put page in edit modus
Click the right block and select 'Setup on child pages'
This is in version 18.104.22.168
But what about if we remove a block? Is it possible to push the "remove" update to existing pages?