WordPress 5.9 compatibility

[jeherve]

WordPress 5.9 is set to release on December 14, 2021 January 25, 2022.
https://make.wordpress.org/core/5-9/

We consequently need to ensure that Jetpack is fully compatible with this new release by Jetpack 10.4, scheduled to be released in early December. by Jetpack 10.5, scheduled to be released in early January.

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• Update our "tested up to" version General: update WordPress version requirements #21788
• Update the version of WordPress required for Jetpack from WP 5.7 to WP 5.8 General: update WordPress version requirements #21788
• Remove WP 5.7 backwards compatibility checks General: update WordPress version requirements #21788
• Refactor Subscriptions block to not use a shortcode (Subscriptions: Remove the shortcode #15227)
• Twenty Twenty Two Compatibility Twenty Twenty Two Compatibility #21449
• Ensure Admin Bar is still functional after https://core.trac.wordpress.org/ticket/28569 lands. Open new issue if there are issues.
• Sync / Themes: take advantage of new delete_theme and deleted_theme introduced in WP 5.8. Sync / Themes: take advantage of new delete_theme and deleted_theme #20261
• FSE + Likes non-functional. Potentially a Core-side fix so need to watch https://core.trac.wordpress.org/ticket/54529
• Test the new language switcher in Login screen and ensure it works well with Protect's Math Fallback as well as SSO. https://core.trac.wordpress.org/ticket/43700
• Miscellaneous block editor changes https://make.wordpress.org/core/2022/01/04/miscellaneous-block-editor-changes-in-wordpress-5-9/
• Miscellaneous Core changes in WordPress 5.9 https://make.wordpress.org/core/2022/01/04/miscellaneous-core-changes-in-wordpress-5-9/
• User Queries: update parameters to take into account changes in WP 5.9 User Queries: update parameters to take into account changes in WP 5.9 #22559

In addition to the above, we can follow the dev-notes that will drop here about the release:
https://make.wordpress.org/core/tag/dev-notes+5-9

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For reference: prior art for WordPress 5.8 compat: #19654

[jeherve]

Ensure Admin Bar is still functional after core.trac.wordpress.org/ticket/28569 lands. Open new issue if there are issues.

This seems it will not land in this release:

Moving to Future Release until the patch is refreshed, with unit tests.

[kraftbj]

5.9 delayed. https://make.wordpress.org/core/2021/11/17/wordpress-5-9-beta-1-delayed/