It doesn't always phone home, that's the thing, a lot of people assume that's the case and that every games built using their engine since the beginning does it but that's not the case.

The way they calculate the installs is not base on that, well unless you enabled Unity Analytics of course, but that's not the case on every games and they are going for every games ever made by their engine.

Maybe, but Unity said it doesn't need to for them to "estimate" an install number.

My theory is that newer installers will troll the registry and try to phone home about anything you did.. That’s the only way you could roll this out without the capabilities for tracking in most present games.

at least on windows.

It does not. They may have plans but you can run wireshark and see it does not, at least for games bought on GoG