As of April 28th, 2023, DHL's Unified Shipment Tracking API started returning 404 statuses for all calls to this endpoint using a tracking number that used to return a 200 status:
https://api-eu.dhl.com/track/shipments?trackingNumber=GM545115623000247160
Figured it out as I was asking:
The service parameter is now required. We track DHL ecommerce solutions so the new endpoint is:
https://api-eu.dhl.com/track/shipments?trackingNumber=GM545115623000247160&service=ecommerce
What's frustrating is that I either missed or never received an email regarding this API change and I could have sworn it wasn't listed as a required parameter before in their API reference. Hopefully this helps anyone else that has this issue.