As I read the RFC[1], every endpoint is supposed to disable the spinning on about 1 in 8 connections. I presume to force middleboxes to accept connections with and without it set.
> Even when the spin bit is not disabled by the administrator, implementations MUST disable the spin bit for a given connection with a certain likelihood. The random selection process SHOULD be designed such that on average the spin bit is disabled for at least one eighth of network paths.
(and as long as there is at least one actor interested in RTT, and this actor happens to controlling both endpoints for a significant amount of traffic crossing the internet (Chrome/Google), there will be good mix of connections with and without spinbit)
> Even when the spin bit is not disabled by the administrator, implementations MUST disable the spin bit for a given connection with a certain likelihood. The random selection process SHOULD be designed such that on average the spin bit is disabled for at least one eighth of network paths.
[1] https://tools.ietf.org/html/draft-ietf-quic-transport-23#sec...