The Maturing of QUIC
The spin bit reminds me of the evil bit[1].  Endpoints/clients/servers don't have much reason to set it, except when explicitly running a trace.  It may work under initial implementations, but I expect it will eventually become a dead field.  Why maintain the code complexity to expose this information to a network operator?  
Even worse, "malicious" clients  could spin the bit randomly.<p>[1] <a href="https://en.m.wikipedia.org/wiki/Evil_bit" rel="nofollow">https://en.m.wikipedia.org/wiki/Evil_bit</a>