It’s affecting everyone - some changes were done on the backend on that day and it has been impacting ESI ever since. Endpoints have slowly getting fixed, but Market endpoints are still degraded.
Most third party devs have talked about the issue on the EVE Discord (#3rd-party-dev-and-esi), and there is even a CSM candidate that is focusing on these issues: Ariel Rin and Third Party Development for CSM19 - #2 by Ariel_Rin