DECNet was a network system for DEC computers, introduced in the mid to late seventies. What physical connector did it use? Was it possible to connect to such a network using just an RS-232 serial port, or was it only for machines equipped with some proprietary port?
DECnet is more of a protocol suite than a physical hardware standard. So asking what kind of physical connector it uses is kind of like asking what kind of physical connector TCP/IP uses -- the answer is, it uses whatever connector you need to use for the particular data link layer you're running DECnet on top of.
If you are running DECnet over Ethernet, then the Ethernet will dictate what kind of connector you need. Thickwire used a vampire tap transceiver. Thinwire used a BNC tee connector. UTP used an RJ-45 connector (the RJ-45 connector survived the upgrade from 10Mbps to 100Mbps Ethernet, and again from 100Mbps to gigabit Ethernet).
If you are running DECnet over a slow serial link, RS-232 is probably the connector you need. Faster links (leased lines, T1 connections, etc.) may have used different connectors but I don't know enough about those to give a detailed answer (last time I used a T1 it was on an RJ-45 connector but I don't know if they always used that or if the RJ-45 replaced an earlier connector).
As already stated, the question misses the point of what DECNet is. This is a common misconception regarding networks.
DECNet sits at the Transport and Network levels of the OSI model of networking. The physical connector is only relevant at the Physical or Data Link levels.
I used DECNet extesively for several years, on Thick-wire, Thin-wire, Optical, Microwave, Dial-up, etc.
DECNet coexists perfectly happily on a physical network with other protocols, such as TCP/IP. Again, I used mixed protocol networks like this all the time.