Appearance -> Template Parts not present for hybrid/classic theme in 6.6-beta-1

[MadtownLems]

Description

I'm testing out 6.6-beta-1, and noticed that Appearance -> Template Parts is not in the main menu

Step-by-step reproduction instructions

The lack of a core-supported theme that meets this description (non-block theme but uses template parts) likely makes this difficult to reproduce, but basically:

1. Create a non-block theme that uses template parts (https://gutenbergtimes.com/block-based-template-parts-for-classic-themes/)
2. Open the dashboard of such a site on 6.6-beta-1, and see that Appearance->Template Parts is not there.

Screenshots, screen recording, code snippet

6.5.3

6.6-beta-1

Environment info

WordPress 6.6-beta-1, no Gutenberg plugin

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

[MadtownLems]

@ndiego helped me see that this is by design, and pointed me here: #52150 (comment)