In the Backends documentation (https://pulumi.io/reference/state/#backends), I read that the web backend is a “free service” with advanced tiers for teams etc. On the Pricing page (https://www.pulumi.com/pricing/), I can only compare the feature set for the paid offerings. Which features are available for the “unlimited individual use”?

The free Community Edition is oriented towards individuals, so none of the "team" or "enterprise" level features are available (organizations, RBAC, SAML/SSO). But everything else is there, no stack/project limits.