uBlock Origin aside, it feels like browsers should have built-in support for dealing with this technique (I'm thinking of Safari content blockers and Chrome declarative net requests). Otherwise it's all too easy for trackers to get around browser protections, rendering content blocking APIs almost useless. You should be able to block the resolved ___domain for a CNAME, not just the source ___domain...
CNAME is a red herring - it would not take that much more effort to delegate the subdomain to a name server controlled by the tracking service or just dynamically update the A/AAAA records yourself.
