I don’t think they would add a default adblocker to Safari. You can’t use any other browser engines, and a system wide ad blocker would seem pretty anti-competitive.
Only to advertisement agencies. And they can go cry up a river.
They stopped tracking by default in safari which is similar.
Now if Google were to do something like add a default adblocker that blocked all non-Google ads... yeah, that'd be anti-competitive.
But if Apple isn't even competing, how can it be anti-competitive?
https://developer.apple.com/support/iad/
Did I miss some other product?