[ArkaPrabhaChowdhury]

The admin menu and admin bar depend on the non-public core-admin icon collection. Unregistering that collection, or one of its icons, currently leaves the corresponding admin UI without icons.

Reject those operations with _doing_it_wrong() and return false before changing either registry. The guards live in the registry methods so both direct calls and the public helpers are covered. Only core-admin is protected: the public core collection and third-party collections remain removable, including cascading removal of their icons.

Regression tests exercise both blocked operations, unchanged SVG output, and successful removal of core icons and their collection. The REST icon test fixture now uses temporary registry instances instead of unregistering the protected collection, and restores the original instances afterward.

Trac ticket: https://core.trac.wordpress.org/ticket/66344

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[nathanrice]

Thinking through edge cases here, if a person wanted to swap core icons for their own, because they were skinning the Admin, this would prevent that, I think.

I don't know how common that would be, but we should probably think through an official "how to" for anyone who may want to do this in the future.

[ArkaPrabhaChowdhury]

Thanks for raising that great edge case point @nathanrice. It makes sense and I have tried to add that functionality. Can you please check if it makes sense and matches expectations?

[nathanrice]

The filter is a good option. One thing to consider (others may disagree, so I'm not insisting here) would be a pre_ filter.

Here's an example from core:

wordpress-develop/src/wp-includes/nav-menu.php

Line 855 in 77393df

$pre_menu_item = apply_filters( 'pre_wp_setup_nav_menu_item', null, $menu_item );

By applying the pre_ filter early and checking for a null value, you can short circuit the function to avoid the unnecessary processing that the function does on the HTML, only for the callback to have to do it all over again.

[ArkaPrabhaChowdhury]

Agreed, an early filter is a better fit for replacing an icon completely. Added pre_wp_get_icon before the registry lookup in 68a7436. Returning null continues normal rendering; returning SVG markup or an empty string short-circuits both the lookup and HTML processing. The callback supplies the complete markup, including any sizing and accessibility attributes.

Kept wp_get_icon for callbacks that modify the rendered SVG.

[nathanrice]

it might be good to pass the $processor instance to the filter too. If a person wanted to transform the html, rather than replace it entirely, it comes with handy helper methods that make this safer. Then, they can just return $processor->get_updated_html() in the callback when they're done.