Are static methods in ASP.NET code-behind classes non-thread-safe?

Yes, you can use static methods - they are thread-safe. Each thread will execute in a separate context and therefore any objects created inside a static method will only belong to that thread.

You only need to worry if a static method is accessing a static field, such as a list. But in your example the code is definitely thread-safe.

nothing shared across threads, so it is thread safe. unless you access static members that other static methods have a chance of executing concurrently with it...