default or selected kernelversion does not have config value CONFIG_DECNET_ROUTE_FWMARK.
Result is shown for kernelversion 5.4.123

DECnet: use FWMARK value as routing key (EXPERIMENTAL)


Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>Network packet filtering (replaces ipchains)
└─>DECnet: use FWMARK value as routing key (EXPERIMENTAL)
In linux kernel since version 2.6.12  
If you say Y here, you will be able to specify different routes for
packets with different FWMARK ("firewalling mark") values
(see ipchains(8), "-m" argument).