The blog states that;
"Catalyst 9600 Series
– Quad supervisor support for virtual stack for greater redundancy", which was echoed some weeks ago by a Cisco SE.

The release notes for 17.1.1 suggests that two Supervisors in a pair of 9606s do not support it, and after some time testing I can confirm 17.1.1 does NOT support SVL- its a feature I was really hoping for and now makes our new core design more difficult and some way less resilient. Can anyone confirm when we can expect it ?