OAuth 2.1 Multi-Tenant Architecture for Carrier Integration: Surviving the 2026 Migration Crisis Without Breaking Tenant Isolation
USPS Web Tools retired on January 25, 2026, and FedEx SOAP services will retire on June 1, 2026. These aren't gradual migrations—they're hard stops forcing thousands of enterprise teams into OAuth 2.1 adoption with mandatory PKCE enforcement. UPS completed their OAuth 2.1 migration