Hi @John_Man ,
If you have a CNAME and use the visitor ID service — your implementation would not be affected.
The s_ecid cookie is essentially used to restore values to the AMCV cookies (MID), which is the cookie Adobe will use to identify visitors.
It came as a response to the Safari ITP situation where client-side cookies are deleted every 7 days.
The AMCV cookie currently relies on the document.cookie API and is set via "client-side." Safari favors cookies that are set from a customer's server therefore it is affected by ITP 2.1.
When safari deletes the AMCV cookie, we can use the ECID cookie to create a new AMCV cookie with the same ID. In addition, the ecid cookie is only set on the domain that matches the tracking server.
Ref: https://experienceleague.adobe.com/docs/id-service/using/reference/ecid-library-methods.html