Ah I was misunderstanding the problem. And learned something new about C#, seems in order to avoid breaking existing code they introduce "contextual keywords" var being added later, it is a contextual. You can create a class 'var' and the compiler will prefer it.

At least in C#, you can define variables with keyword names like this:

var @struct = "abc"

I think in Kotlin you can do the same, and even include spaces with backticks like val abstract class = "abc"

I'm not sure if other languages allow that, regardless it should be rarely used.

any new keyword could break backwards compatibility

Wouldn't that happen anyway with variable and function names? Any type other than primitive/built in ones are usually camel case so lower case keywords are more likely to clash with single word variable and function names, unless you restrict the cases of those too or allow keyword overriding or something.