We have been evaluating using ECID with an AppMeasurement tracking project that is in progress. We want to make that our new tracking created from this project is as future proof as possible. We have multiple domains involved with our websites (mainwebsite.com with a separate bookingdomain.com for purchases) so it is critically important that cross domain tracking work accurately and that visitors are properly linked together across domains. At a high level we have come to the below findings but want to make sure from an Adobe ECID expert (who is familiar with ITP and recent updates) that we are making the optimal choices.
Current direction based on our research
Move forward with ECID and use the appendVisitorIDsTo function to allow our cross domain tracking to work.
CNAME – it seems that it would be optimal to use a data collection CNAME for our website domains.
demdex.com and demdex cookie – this is a third party cookie and is blocked by Safari and Firefox (ITP) which could break our cross domain tracking?
Will link decoration using the appendVisitorIDsTo function be blocked by browsers in the future and will this function work properly today in Safari/Firefox?
Will using CNAMEs help our cross domain tracking be accurate in browsers that block third party cookies like Safari and Firefox? We have seen inconsistent articles that suggest this may or may not help.
Append appendVisitorIDsTo Function is the only method I would say that can persist the visit from one domain to another domain when the browser blocks the third party cookie. Check out my take on Google Recent announcement to 'phase out' third party cookies on Google Chrome and its impact on Adobe Analytics Cross-Domain Tracking: Reference Link. This might give you a clear idea!
So, use the method and go ahead with your implementation.
I think this might be an issue with cross domain tracking via CNAME below. It's from the 3rd link you posted. I had the same question myself.
If you are also using this same data collection server on other sites (for example, myothersiteA.com, and myothersiteB.com), and a visitor later visits these sites, the cookie that was set during the visit to mymainsite.com is sent in the HTTPS request to the data collection server (remember that browsers send all cookies for a domain with all HTTPS requests to that domain, even if the domain doesn’t match the domain of the current website). This is what is known as using a cookie in a third-party context, or just a third-party cookie, even though you are using a CNAME. Adobe recommends a CNAME for each unique domain.
Note: Safari blocks all cookies in the third-party context regardless of how they are set.
I think your best bet is to use Visitor.appendVisitorIDsTo - that's what I have inferred from the many different references, and what Adobe shared with me.