Hmmm. That's a bummer that Shopify isn't being responsive. Here's what I have found:
Can Shopify Plus acts as an Identity Service provider (physically store the users without using 3r party ISP) and allow other applications (including ours) to authenticate via SAML?
Yes, documented here: https://help.shopify.com/en/manual/shopify-plus/security/saml
This looks like this only works with Shopify users who are in your organization. (Employee time tracking, not time recording.)
Can Shopify (Plus or Non-Plus) authenticate users using Shopify accounts.
I don't know. This https://shopify.dev/tutorials/authenticate-with-oauth sure looks like a OIDC flow, but I'm not sure how it works without setting it up. Have you tried to set up an OIDC identity provider? That's what I'd do.
Dan, You right
It`s works without setting it up, Thank You