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This block is the legacy custom HTML and as far I know, it’s hidden but you
add_filter( 'widget_types_to_hide_from_legacy_widget_block','sof_hide_legacy' );
function sof_hide_legacy( $widget_id ){
$widget_to_show = array( 'custom_html' );
$widget_id = array_diff( $widget_id, $widget_to_show );
return $widget_id;
}
Does this help you ?