Gutenberg: Autosave errors out on custom post type that don't support revisions

[jonathanbardo]

Describe the bug
When editing a custom post type that doesn't support revisions, autosaving is triggered hits a 404 since the REST endpoint autosave doesn't exist.

To Reproduce
Steps to reproduce the behavior:

1. Create a custom post type without revisions support
2. Create a new post on this custom post type
3. Write a few lines to trigger autosaving
4. See red banner error and 404 in network console.

Expected behavior
I would have expected the autosave not to happen if the custom post type doesn't support revisions.

Additional context

• Gutenberg v3.0.1

[aduth]

I would have expected the autosave not to happen if the custom post type doesn't support revisions.

Within this, it may be good to clarify:

• Should there be no delayed save whatsoever for these post types (i.e. only explicit "Save" actions from the user)
• How is previewing expected to work, particularly for published posts? (How does it work in the Classic Editor?)

[danielbachhuber]

Related #7442

I'm thinking that maybe autosaves should _always_ exist, not just on post types that support revisions.

[adamsilverstein]

In the classic editor, autosaves continue to work regardless of whether the post type supports revisions. Turning off revisions, explicitly still allows for a single autosave revision (per user), see https://codex.wordpress.org/Revisions#Revision_Options