When you performed the migration Xcode asked about @objc inference and you probably selected the new type instead of Swift3.

Possible solutions:

Use @objc

Use @objc on each method, as needed instead of the whole class.

Use @objcMembers

You could just use @objcMembers on the class.

Applying @objcMembers attribute to a class implicitly adds the @objc attribute to all of its Objective-C compatible members.

Writing Swift Classes and Protocols with Objective-C Behavior

Keep in mind: because applying the @objc attribute can increase the compiled size of an app and adversely affect performance, only apply the @objcMembers attribute on declarations when each member needs to have the @objc attribute applied.

Switch inference to the old behavior

You can also change the project’s behavior under:
Build Settings > Swift 3 @objc Inference > On/Off

Related questions:

• The use of Swift 3 @objc inference in Swift 4 mode is deprecated?
• How can I deal with @objc inference deprecation with #selector() in Swift 4?