default or selected kernelversion does not have config value CONFIG_DECNET.
Result is shown for kernelversion 6.5.1

DECnet Support

modulename: decnet.ko

configname: CONFIG_DECNET

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>DECnet Support
In linux kernel since version 2.6.12  
The DECnet networking protocol was used in many products made by
Digital (now Compaq). It provides reliable stream and sequenced
packet communications over which run a variety of services similar
to those which run over TCP/IP.

To find some tools to use with the kernel layer support, please
look at Patrick Caulfield's web site:

More detailed documentation is available in

Be sure to say Y to "/proc file system support" and "Sysctl support"
below when using DECnet, since you will need sysctl support to aid
in configuration at run time.

The DECnet code is also available as a module ( = code which can be
inserted in and removed from the running kernel whenever you want).
The module is called decnet.

source code:

is selected by