ROUTE_MPATH was added to the GENERIC kernel in r368648.
According to the plan in D27428, it was enabled with net.route.multipath sysctl set to 0.
The rationale behind it was to minimise the potential problems arising from both compiling in functionality and turning it on.
Given enough time has passed, this change enables route multipath by default.
The goal is to ship FreeBSD 13 with multipath turned on.