Enable link color in classic themes

functions.php

<?php

add_filter( 'wp_theme_json_data_theme', function ( $theme_json ) {
	$theme_json->update_with( array_merge(
		$theme_json->get_data(),
		[
			'settings' => [
				'color' => [
					'link' => true,
				],
			],
		],
	) );

	return $theme_json;
} );