Newsletter: enable modernized dashboard by default

[dhasilva]

Closes #48530

Proposed changes

• Release the modernized Newsletter dashboard by flipping the default value of two feature filters from false to true at every call site:
  • rsm_jetpack_ui_modernization_newsletter — the canonical gate for the wp-build Newsletter dashboard. Now defaults on in Newsletter\Settings::is_modernized(), the wpcom/v2/subscribers/* REST route registration, the subscriptions module menu, and the jetpack-mu-wpcom admin menu (the legacy Calypso "Subscribers" submenu is retired).
  • jetpack_wp_admin_subscriber_management_enabled — now defaults on in the Newsletter script data, the subscriptions module menu, and the Jetpack Redux state.
• Hosts that need the legacy experience can still opt out with add_filter( '<flag>', '__return_false' ).
• Updated the related docblocks/comments and inverted test_is_modernized_defaults_to_false → test_is_modernized_defaults_to_true.
• The Backup, VideoPress, and Scan MODERNIZATION_FILTER constants are separate flags and are intentionally untouched.

Related product discussion/links

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Spin up a site with this branch (no filter overrides).
• Go to Jetpack → Newsletter in wp-admin and confirm the modernized wp-build dashboard loads with the Subscribers / Settings tabs (rather than the legacy Settings-only screen).
• Confirm the standalone Calypso Jetpack → Subscribers submenu is no longer present.
• Confirm GET /wpcom/v2/subscribers/list is registered (returns a 401/data response, not rest_no_route).
• Add add_filter( 'rsm_jetpack_ui_modernization_newsletter', '__return_false' ); (and/or jetpack_wp_admin_subscriber_management_enabled) via a snippet and confirm the legacy behavior returns — proving the opt-out still works.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the release/newsletter-dashboard-default-on branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack release/newsletter-dashboard-default-on

bin/jetpack-downloader test jetpack-mu-wpcom-plugin release/newsletter-dashboard-default-on

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen as soon as you deploy your changes after merging this PR (PCYsg-Jjm-p2).
• WoA releases happen weekly.
• Releases to self-hosted sites happen monthly:
  • Scheduled release: June 2, 2026
  • Code freeze: June 1, 2026

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/newsletter/src/class-settings.php	48/188 (25.53%)	1.06%	-2 💚

Full summary · PHP report · JS report

[keoshi]

Works great in my testing!

One thing I am seeing in the header is a shift in height when switching between the two tabs:

[dhasilva]

That tab shift issue is due to the use of AdminPage. The other modernized dashboard use the same component, so by standardizing it I can find a general fix now. Will work on it today.