Skeleton for filtering theme.json for classic themes at all layers

lib/compat/wordpress-6.1/hook-theme-json-for-classic-themes.php

@@ -0,0 +1,46 @@
+<?php
+
+function theme_json_current_theme_has_support( ){
+	return WP_Theme_JSON_Gutenberg::has_theme_support();
+}
+
+function theme_json_default_filter_for_classic_themes( $theme_json_data ) {
+	if ( theme_json_current_theme_has_support() ) {
+		$new_data = array(
+			'version'  => 2,
+			'settings' => array( /* we should maintain the presets by core */ ),
+			'styles'   => array( /* clear this? add only the base-layout-styles? */ ),
+		);
+		$theme_json_data->update_with( $new_data );
+	}
+	return $theme_json_data;
+}
+add_filter( 'theme_json_default', 'theme_json_default_filter_for_classic_themes');
+
+function theme_json_blocks_filter_for_classic_themes( $theme_json_data ) {
+	if ( theme_json_current_theme_has_support() ) {
+		$new_data = array(
+			'version'  => 2,
+			'settings' => array(),
+			'styles'   => array( /* add button styles for classic here */ ),
+		);
+		$theme_json_data->update_with( $new_data );
+	}
+	return $theme_json_data;
+}
+add_filter( 'theme_json_blocks', 'theme_json_blocks_filter_for_classic_themes' );
+
+function theme_json_reset_for_classic_themes( $theme_json_data ) {
+	if ( theme_json_current_theme_has_support() ) {
+		$new_data = array(
+			'version'  => 2,
+			'settings' => array(),
+			'styles'   => array(),
+		);
+		$theme_json_data->update_with( $new_data );
+	}
+
+	return $theme_json_data;
+}
+add_filter( 'theme_json_theme', 'theme_json_reset_for_classic_themes' );
+add_filter( 'theme_json_user', 'theme_json_reset_for_classic_themes' );