Add a checkbox to toggle the "Unfiltered html" capability for Administrators in a multisite.

unfiltered-html-toggle.php

<?php
/**
 * Add a checkbox to toggle the "Unfiltered html" capability for Administrators in a multisite.
 */

/**
 * Re-enable unfiltered_html for Administrators only on a Multisite.
 * 
 * @see https://core.trac.wordpress.org/browser/tags/5.8/src/wp-includes/capabilities.php#L421
 */
add_filter(
	'map_meta_cap',
	function( $caps, $cap, $user_id ) {
		$user = get_userdata( $user_id );

		if ( empty( $user->roles ) ) {
			return $caps;
		}

		$can_insert_iframe = get_user_meta( $user_id, 'admin_insert_iframe', true );

		if ( 'unfiltered_html' === $cap && in_array( 'administrator', $user->roles, true ) && $can_insert_iframe ) {
			$caps = array( 'unfiltered_html' );
		}

		return $caps;
	},
	10,
	3
);

/**
 * Insert a new checkbox in the User Profile page.
 */
add_action(
	'personal_options',
	function( $profileuser ) {
		// This option is only available if current logged-in user is a Super Admin
		// and user being edited is an Administrator.
		if ( is_super_admin() && in_array( 'administrator', $profileuser->roles, true ) ) {
			$can_insert_iframe = get_user_meta( $profileuser->ID, 'admin_insert_iframe', true );
			?>
			<tr class="show-admin-bar user-admin-bar-front-wrap">
				<th scope="row"><?php esc_html_e( 'Add unfiltered HTML', 'edcwm' ); ?></th>
				<td>
					<label for="admin_insert_iframe">
						<input name="admin_insert_iframe" type="checkbox" id="admin_insert_iframe" value="1"<?php checked( $can_insert_iframe ); ?> />
						<?php esc_html_e( 'Allow user to add restricted tags like iframe and embed into a page/post.', 'edcwm' ); ?>
					</label><br />
				</td>
			</tr>
			<?php
		}
	}
);

/**
 * Save custom data to user meta.
 * 
 * @param int $user_id User ID.
 */
function edcwm_save_custom_user_meta( $user_id ) {
	if ( ! wp_verify_nonce( $_REQUEST['_wpnonce'], 'update-user_' . $user_id ) ) {
		// Security check fail. Exit early.
		return;
	}

	$can_insert_iframe = filter_var( isset( $_POST['admin_insert_iframe'] ) ? $_POST['admin_insert_iframe'] : 0, FILTER_SANITIZE_NUMBER_INT );
	update_user_meta( $user_id, 'admin_insert_iframe', $can_insert_iframe );
}
add_action( 'personal_options_update', 'edcwm_save_custom_user_meta' );
add_action( 'edit_user_profile_update', 'edcwm_save_custom_user_meta' );

User profile page