Are Java and C# interface common enough to justify using the same tag? Otherwise maybe two tags java-interface and C#-interface would solve the problem. I see no purpose of the tag outside these specific language keywords.

AFAIK, "interface" is virtually the same thing in C# and Java. Not sure about other languages that seem to use the concept (e.g. ADA - https://www.adaic.org/resources/add_content/standards/05rat/html/Rat-2-4.html ). I guess that multiple languages that do not support multiple inheritance needs the concept or something similar.

Anyway, I think c#-interface and java-interface are better choices than the current options. We can easily define new "interface" tags for other languages, if needed.

Are any of these tags worth it? Who's searching for questions related to interface-the-abstract-concept or interfaces-as-occurring-in-c#, in our community of 589 posts?

r~~‭ As referenced in my post, the (language agnostic) concept of interface is quite heavily used on SO. It is also an important concept used for most applications developed in C# or Java.

The number 2 is also an important concept used in many software applications but it'd be madness to suggest a tag for it, because who searches for ‘questions related to 2-ness’?

I don't know if SO is doing the right thing and I'm asking under what circumstances would a user (here or there) say, what I need today is a list of all questions having to do with (OOP) interfaces? We have tags for languages because people have interest or expertise in languages; likewise frameworks, design patterns, etc. Who is an ‘interface’ enthusiast?