Most API deprecations announce themselves. You pin a version, the vendor retires it, and your next call comes back 410 Gone with a link to the migration guide. Annoying, but it's a signal — something in your monitoring goes red, someone gets paged, and the work gets scheduled.
Klaviyo does something different, and it's written plainly in their versioning policy:
If a retired revision date is passed to the request header, Klaviyo falls forward and responds to your request with the same behavior as the next oldest revision.
Read that again with an on-call hat on. Your revision: 2024-07-15 header keeps getting sent. Klaviyo keeps returning 200. Your logs, your traces, your API client's user-agent string — all still say 2024-07-15. But the code path serving you changed.
Revision 2024-07-15 reached its support end date on July 15, 2026. If you're pinned to it — or anything older — you've been served 2024-10-15 behavior since then. And 2024-10-15 itself retires on October 15, 2026, which is a sharper cliff than the one that already passed.
