For the MCS today, we default to serving a spec v2 ignition if there is no header (normally provided by the ignition binary) indicating what version to serve. This was for historical purposes to ease transitioning from spec 2 to spec 3, so existing workloads that directly request ignition (I think windows and rhel back then) didn't break.

Now that we are many versions deep into spec 3, it probably makes sense to switch that default.

Also see: https://bugzilla.redhat.com/show_bug.cgi?id=2037626