-From one hand I would suggest to think of OpenStack release<->major Fuel release has pretty much 1 to 1 relation.
See https://www.mirantis.com/software/openstack/releases/ for that ‘matrix’ thingy.
From the other - yeah, you can maintain your older environments using newer Fuel to some extent.
They will remain operationa and the statements in recent Fuel releases is like:
"Fuel can manage environments that were deployed with (x.x, check exact release) releases, assuming that you created the environment with the earlier release and upgraded the Fuel Master node rather than doing a fresh install. “
But me - I am not sure about lifecycle activities involving Fuel (expansion/replacement)
for MOS 8.0/MOS 9.0 there are things like 'Integrate RHEL Compute Nodes
However the support of that is not really for deployment (unlike Ubuntu and in certain releases CentOS)
Side note: Mirantis website has become a bit of mess, specifically when it comes to documentation.
Yeah, I understand that MCP/updated look-and-feel of website is new and exciting, but still…