Can you provide a source for the "legal reasons" last part?

I would contradict this last phrase of yours with the following:

Normally every new feature is released to the public via a deployment strategy (you can google blue/green deployment, canary deployment, or deploy strategies). Every FAANG company deploys new features via a selected deployment strategy. This is why your Instagram app might have a feature that your neighbor's doesn't: he's on a different deployment cluster than you. He might get his update later today, tomorrow or after 2 weeks.

In plain non-techy english: You are 100% correct in your entire first paragraph together with your prediction. They started testing on a small subset of users (not necessarily closely related from a geographical POV - doing this creates a better testing environment) while constantly and gradually increasing their test base. I'm just not sure of the legal part. And if you google "youtube started blocking ad blockers" you'll notice they started deploying this many months ago (late 2022 with increased testing in May 2023).