I cannot find a first-party source of why and when Google is shutting down domain fronting, but it makes sense from a cybersecurity perspective. Domain fronting is widely used by malware [1, 2] to evade network-based detection.

[1] https://www.fireeye.com/blog/threat-research/2017/03/apt29_d... [2] https://www.cyberark.com/threat-research-blog/red-team-insig...