The product provides login and sign up with Slack which means all product plans include SSO under whatever scheme the Slack workspace is provisioned with.
The SAML feature in the Pro and up plans is about hooking us up directly to your identity provider to do things like RBAC and group syncing, and we pay WorkOS a monthly fee of several hundred dollars per customer that uses this, so makes sense to gate in the higher plans.
Many Cyber Insurance policies mandate SSO