For a given technology, ask three questions:
Is the technology still considered "state of the art"? In other words, has something better been sold or freely available?
Does the original manufacturer/author still support the technology?
Is there another StackExchange site that is willing to discuss the technology?
In my humble opinion, if the answers to all three questions are "no", then it is appropriate for Retrocomputing.
The answers are all "no" for Digg's web APIs.
Update:
I'm not 100% sure question #2 is necessary.
To answer @wizzwizz's comment, here are some examples:
8085 processor: NNN --> retro
80386 processor: NNN --> retro
Intel Core Solo: NNY --> not retro
6805 microcontroller: (still used for automotive applications) NYN --> not retro
Wired Ethernet: (Xerox is OEM) YNY --> not retro
WiFi: YYY --> not retro
LocalTalk: NNN --> retro
MS-DOS: NNN --> retro
Windows XP: NNY --> not retro
Windows 10: YYY --> not retro
It's really hard to come up with a good YNN example. If something is still state-of-the-art, it is usually still being sold and supported by the OEM, and there is often a SE site willing to discuss it.
The idea is that NNN should qualify as being on-topic. However, there also needs to be a list of topics that are explicitly allowed, even if they have some "yes" answers. Some examples of things that should be explicitly on-topic:
Processors with less than 32 bits. Some are NYN, NNY, or NYY.
Aspects of serial/UART/RS-232 communication not covered by other SE sites (NYN).
System V or BSD Unix (some support on various SE sites).
Microsoft Windows versions prior to XP.
Classic MacOS (some support on Think Different).
USENET (NYN).
Adapting an old (on-topic) technology to use a new (off-topic) technology. For example, Ethernet on a Mac Plus.