Users of classic themes can access and use parts of the Site Editor but its a broken experience

[carolinan]

Description

If I activate Gutenberg Version 20.1.0 or the current trunk and a classic theme like Twenty Ten and navigate to wp-admin/site-editor.php?p=%2Ftemplate I am met with the template screen from which I can create new block templates. I can't use the templates.
If I try to add content to a page template I get the message "Updating failed. You are probably offline."

The page screen wp-admin/site-editor.php?p=%2Fpage is also available.

I was expecting both screens to show "The theme you are currently using is not compatible with the Site Editor."

Step-by-step reproduction instructions

Activate Gutenberg
Activate a classic theme such as Twenty Ten, Hello Elementor
Visit the different Site Editor screens:
wp-admin/site-editor.php?p=%2Ftemplate
`wp-admin/site-editor.php?p=%2Fpage

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure

[Rishit30G]

Thanks for sharing the issue,

I was successfully able to reproduce the issue, and yes these wp-admin/site-editor.php?p=%2Ftemplate and
wp-admin/site-editor.php?p=%2Fpage links shouldn't be visible on T10 theme and should show "The theme you are currently using is not compatible with the Site Editor."