Description of the need
I've just implemented hook_theme_registry_alter() in a custom block and checked the registry content to see long list of variables, including for unused layout templates. Disabling them on admin/structure/layouts/settings freed registry from unnecessary content and even-though human eyes might not notice, definitely made things slightly faster.
Proposed solution
I believe Backdrop must be shipped as performant as possible, so I suggest to disable all unused layout templates by default. Users can easily enable them if they need additional templates.
Additionally, we could add a pointer link on pages like admin/structure/layouts/manage/dashboard/configure as shown below:

Recent comments
Hi Kevin I am interested assisting you developing a theme by cloning feature from existing WordPress website. Please let me know your suitable time to discuss further...
Create a theme from existing website
I've updated the Zulip link in both places I found it. No need to post again, unless you have something new to say. We'll pull together feedback from all the sources.
Backdrop CMS Core Priorities
Should we post here again, what we posted over there? Or would that unnecessarily duplicate things? The link to a Zulip thread in this initial post leads to an internal one, but there's...
Backdrop CMS Core Priorities