Twenty Twenty-One: Consider removing IE support

[sabernhardt]

Parent: #64278

Twenty Twenty-One added CSS and polyfill scripts to support IE11, but Microsoft dropped their support for Internet Explorer.

[sabernhardt]

According to ​StatCounter, IE visits increased for July and August last year, but the numbers total about 0.1% most months. (I used the "Download Data (.csv)" link.)

Month	IE 11.0	IE 9.0
2024-12	0.09	0.01
2025-01	0.08	0.01
2025-02	0.09	0.01
2025-03	0.1	0.01
2025-04	0.1	0.01
2025-05	0.09	0.01
2025-06	0.11	0.01
2025-07	0.29	0.01
2025-08	0.35	0.02
2025-09	0.14	0.02
2025-10	0.06	0.01
2025-11	0.09	0.01
2025-12	0.08	0.01
2026-01	0.06	0.01

• Removes polyfill scripts, but continues to register their handles.
• Removes IE-specific styles from the main stylesheet.
• Removes the separate IE stylesheets from the build process and from enqueuing functions.
• Removes or replaces contents of the unused files.
• Removes the is-IE class.

Props: iflairwebtechnologies

Trac 64590

[joedolson]

I want to commit this before #64375.

I think this can be committed as is; but I wonder if we want to also remove the contents of the IE skiplink fix on line 470 of functions.php and it's source file?

The function hasn't been in default usage since https://core.trac.wordpress.org/changeset/55861, but I think we could probably remove it entirely as part of this.

@sabernhardt Let me know if you agree.

• Replaces contents of skip-link-focus-fix.js.
• Outputs an HTML comment if anyone adds twenty_twenty_one_skip_link_focus_fix to the wp_print_footer_scripts action (in a plugin or a child theme).

Trac 64590

[joedolson]

Marking for commit; will commit the original PR, then reopen for the follow-up, pending additional approval.

[joedolson]

In 61628:

Twenty Twenty One: Remove support for Internet Explorer.

No version of Internet Explorer is still supported by WordPress or Microsoft. Usage of Internet Explorer has dropped to about 0.1%.

Remove IE-specific CSS and polyfills.

Props sabernhardt, westonruter, mukesh27, joedolson.
Fixes #64590.

[joedolson]

Re-opening for follow-up to remove the IE-specific Skip Link focus fix.

Closed in https://core.trac.wordpress.org/changeset/61628

[joedolson]

In 61635:

Twenty Twenty One: Remove Skiplink shim for Internet Explorer.

No version of Internet Explorer is still supported by WordPress or Microsoft. Usage of Internet Explorer has dropped to about 0.1%.

Follow up to [61628] to also remove JS that fixed skip link behavior for IE. Also remove overlooked contents of ie-editor.css.map.

Props sabernhardt, westonruter, joedolson.
Fixes #64590.

Closed in r61635