As it stands no one cares about DECnet any more. Red Hat's Christine Caulfield flagged the code as orphaned in 2010. VMS was the last even mainstream OS to use it.
Historically it is a relic from the days when TCP/IP is not the only network protocol on the block. Linux used to support multiple network protocols, such as Novell's IPX protocol, SPX, and a few others whose names we have forgotten. TCP/IP was just an optional extra 1993 n both Windows for Workgroups 3.11 and Windows NT 3.1.
DECnet supplied PathWorks lets DOS, Windows, and Mac clients chat to VAX servers. All over the world WANs ran over DECnet.
But now they don't the protocol wars are finally over.TCP/IP won in the end.