Though according to caniuse, beacons is fully implemented which has much the same effect from a tracking PoV.
As a side note, I was somehow completely unaware of that feature (and I don't think in a "heard about it and forgot" sort of way). Every day is a school day...
On the mdn page for it they say there are other less legitimate techniques like adding an image which will start loading and delay the page unloading. So if they didn't add beacons it would mean other less performant ways would be (ab)used.
As a side note, I was somehow completely unaware of that feature (and I don't think in a "heard about it and forgot" sort of way). Every day is a school day...