Advanced routing protocols like BGP, OSPF, IS-IS, VXLAN and EVPN on platforms without a paid support option (like Quagga, Mikrotik, FRR).
To me it feels 'wrong' that questions about these protocols, which this people on SE should be able to answer best, are considered off topic only based on the fact that there's no commercial support.
Currently off topic, ...
Cloud-based networking, such as Amazon VPC configuration, NAT gateways, VPN configurations, BGP routing, load balancing, etc. Also Azure virtual networking, and similar offerings by other cloud providers.
Admittedly, there will be overlap with questions on the basic compute and application services these vendors offer, and those would best be answered on ...
Services that are required for normal operation of a network such as DHCP, DNS, RADIUS, etc.
Currently mostly off topic (unless on a platform such as IOS), and should be on topic.
Other related meta discussions: here, here, and here.
Clarification of "protocols above L4 in the OSI model are off-topic" to specifically mention routing and tunnelling protocols, which should all be on-topic.
L2TP is on top of UDP
BGP is on top of TCP
I don't know that anyone would disagree: I'm just trying to make the rules clearer.
I'd like to see virtual host switches (VMware ESXi and MS Hyper-V) added to the 'on' topics.
These switches share a lot of logic with their physical cousins - and need to interface with them - but do have their own flavor.
On-topicness (is that a word?) should be limited to the switch configuration on the host, not including other host configuration items.
Overlay networks, such as torrent or bitcoin, should be explicitly off-topic.
There have also been questions about other proprietary or open source overlay networks, but this topic has not been explicitly addressed in the past.
Linux based and/or open source products often used as routers/gateways/access control for small to medium business (or larger enterprises in niche conditions).
These come to mind quickly as possible examples, but this is not a definitive list: Vyatta, MikroTik, pfSense, PaketFence, or OPNsense.
How to define the scope of such products would have to be a ...
This morning's tally on https://networkengineering.stackexchange.com/:
48 total (on first screen)
I came here to try to grow my understanding of Linux Networking. But now I can see that even if I qualify in the future to ask a Q here, I won't be back. I think this is just too much of an unfriendly group here. There ...
Questions should be on topic by their actual topic and not their subject. If someone by dumb luck happens to replace "home" with "office" in their question, then people here consider it on topic, when that is a completely irrelevant data point.
If someone asks a question about ARP proxying on their Cisco, that is no more valid than if they asked that about ...
As was mentioned in a comment, you could try asking this on https://www.serverfault.com for a business network, although I'm not sure if it's a good fit there. You should check their help center to see what type of questions are on topic there.