sendBeacon() has been available as described in https://www.chromestatus.com/feature/5517433905348608. A part of sendBeacon()'s functionalities has been temporarily disabled later due to a security issue. The change is that, from M60, sendBeacon() will throw when called with a Blob whose type is not a CORS-safelisted value for the Content-Type request header as the data argument. This entry is used for tracking progress on resolving the issue and re-enabling the functionality.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon