> completely impossible
There were many 3rd party browsers on the App Store with ad blocking support since iOS 4 or earlier.
But no way to set them as the default browser so links that open up from apps were still opening in Safari with the ads.
Jailbreak made it possible, or you could set up an ad-blocking proxy on your network.
Yes or now any of dozens of apps in the App Store which is what makes it a turning point.
It wasn't impossible. There were basically two solution up until now: use an alternative browser with ad blocking included (e.g. iCab), or a VPN that filters ads for you.
Or a DNS server that blocks ad network domains
This isn’t sufficient, you’d still get empty squares everywhere on the pages with ads.
Because iOS was the last major platform on which true adblocking was (AFAIK) completely impossible until this week.
Desktop browsers have had adblocking for ages, and even non-rooted Android devices can block ads in webpages (root is required only to block ads in native apps).