What counts as mainstream for you?

Java has recently added sealed classes/interfaces which offer the same features as sum types, and I would argue that Java is definitely mainstream.

Kotlin has a similar feature. It might be used less than Java, but it's the default language for Android.

Swift has `enum` for sum types and is the default language for iOS and MacOS.

Likewise for Rust, which is gaining traction recently.

Typescript also has union/sum types and is gaining lot of traction.