Magento 2.3 : How to create product attribute via declarative schema?

I wrote an entire blog post about this. It is quite extensive and goes into detail about all of the intricacies: https://markshust.com/2019/02/19/create-product-attribute-data-patch-magento-2.3-declarative-schema/

That said, here is the final class of code you can work with:

<?php
namespace Acme\Foo\Setup\Patch\Data;

use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\ModuleDataSetupInterface;
use Magento\Framework\Setup\Patch\DataPatchInterface;

class AddAlternativeColorAttribute implements DataPatchInterface
{
    /** @var ModuleDataSetupInterface */
    private $moduleDataSetup;

    /** @var EavSetupFactory */
    private $eavSetupFactory;

    /**
     * @param ModuleDataSetupInterface $moduleDataSetup
     * @param EavSetupFactory $eavSetupFactory
     */
    public function __construct(
        ModuleDataSetupInterface $moduleDataSetup,
        EavSetupFactory $eavSetupFactory
    ) {
        $this->moduleDataSetup = $moduleDataSetup;
        $this->eavSetupFactory = $eavSetupFactory;
    }

    /**
     * {@inheritdoc}
     */
    public function apply()
    {
        /** @var EavSetup $eavSetup */
        $eavSetup = $this->eavSetupFactory->create(['setup' => $this->moduleDataSetup]);

        $eavSetup->addAttribute('catalog_product', 'alternative_color', [
            'type' => 'int',
            'label' => 'Alternative Color',
            'input' => 'select',
            'used_in_product_listing' => true,
            'user_defined' => true,
        ]);
    }

    /**
     * {@inheritdoc}
     */
    public static function getDependencies()
    {
        return [];
    }

    /**
     * {@inheritdoc}
     */
    public function getAliases()
    {
        return [];
    }
}