Think of it as an iceberg & Chrome users as a boat.
Assuming no changes, this is landing in Chrome Canary now, so we're watching the Chrome Canary boat hit the iceberg. The Chrome Beta boat is going to hit in a few weeks. Finally the Chrome Stable boat is scheduled to hit in mid November.
Now Google may choose to hold back actually enabling this flag immediately. It wouldn't be the first delay. But likely in mid November is when all the posts will start to appear of people asking where their ad blocker went.
(Although I'm guessing it actually is delayed until after the holidays and in the new year, but that's just wild speculation.)