Menus: Deprecate block_core_navigation_submenu_render_submenu_icon() rather than removing it

[ecairol]

Summary

block_core_navigation_submenu_render_submenu_icon() was removed from src/wp-includes/blocks/navigation-submenu.php in [60810] (Gutenberg PR #74853, synced via wordpress-develop PR #10865) as part of consolidating submenu icon rendering into the new shared helper block_core_shared_navigation_render_submenu_icon().

The removal landed without a _deprecated_function() notice or a backwards-compatible shim, so any theme or plugin that previously called the function — for example from a render_block_core/navigation-submenu filter to customise the dropdown chevron — now triggers a fatal PHP error on 7.0.

This PR restores the function as a thin deprecation shim:

• Triggers _deprecated_function() so callers see actionable guidance to migrate to block_core_shared_navigation_render_submenu_icon().
• Returns the output of the new helper, preserving the original rendered markup.
• Includes a phpunit test asserting the deprecation notice fires and the return value matches the new helper.

Trac ticket: https://core.trac.wordpress.org/ticket/65287

Test plan

• php -l src/wp-includes/blocks/navigation-submenu.php passes (verified locally).
• phpunit --filter Tests_Blocks_NavigationSubmenuRenderSubmenuIcon passes.
• On a site whose theme calls block_core_navigation_submenu_render_submenu_icon(), the page renders without a fatal and a deprecation notice is logged.
• Existing Navigation Submenu rendering is unchanged (the shim is only used by external callers).

Props ecairol.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props ecairol.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[ecairol]

Closing — wrong repo for this fix.

CI surfaced the actual situation: src/wp-includes/blocks/navigation-submenu.php is now overlaid by the gutenberg:copy build step from the Gutenberg release, so a wordpress-develop-only change to that file fails git diff --exit-code. The canonical fix has to land in WordPress/gutenberg first and propagate via the Gutenberg → Core sync.

Re-opening as a Gutenberg PR against packages/block-library/src/navigation-submenu/index.php. Will link the new PR back to Trac #65287.

[ramonjd]

Re-opening as a Gutenberg PR against packages/block-library/src/navigation-submenu/index.php. Will link the new PR back to Trac #65287.

Good call. I guess the deprecation wrapper can be packages/block-library/src/navigation-link/shared/render-submenu-icon.php. Same fix as here.

Thank you for handling this!

[ramonjd]

I guess the deprecation wrapper can be packages/block-library/src/navigation-link/shared/render-submenu-icon.php.

I withdraw that. What you've done in WordPress/gutenberg#78484 LGTM. Thanks again