Is shared hosting ever enough for a real online store?

Yes, for a smaller catalogue and moderate traffic it handles it fine - the mistake is staying on it after growth, not starting on it.

What is the clearest sign I have outgrown shared hosting?

Checkout slowing down or timing out during busy periods, not just the site feeling generally slower - that specifically points to shared resources being stretched.

Does the platform I use (Shopify, WooCommerce, Salla) change the hosting answer?

Heavily. A self-hosted platform like WooCommerce needs real server resources you control; a hosted platform like Shopify or Salla handles that layer itself.