It's not exactly 'opt-out', they ask you on first boot or after major upgrades, and you either select "Share with Apple" or "Don't Share with Apple". It's just that the "Share" button is coloured blue so looks more default since it's more prominent (at least on iOS, I think it's basically the same on macOS).

It's not like it's enabled by default and you have to know to go and find the setting to turn it off or anything..

It’s opt-out, but it’s not enabled silently. It’s a pre-ticked checkbox on a screen you have to review when you first setup the machine (and when you do a major version OS upgrade).

IMO that’s quite different to something that’s just silently on by default, and requires you to set an environment variable or run a command to opt out.

No the default action is to do nothing (ie do not install the OS). You have to actively consent or reject.

Yeah, that's kind of surprising, given that Apple is often hailed as a privacy champion.

Well, Apple generally has so much info about your every step people stopped caring a long time ago.