Does it get free replacement butterfly keyboard

Click on the following: Keyboard Service Program for MacBook, MacBook Air, and MacBook Pro - Apple Support

The 2016 MacBook Retina is on that list but it may be excluded because of the following in that document:

The program covers eligible MacBook, MacBook Air, and MacBook Pro models for 4 years after the first retail sale of the unit.

This is primarily a user-to-user support forum; other Apple users like you. Contact Apple Support.

I don't believe that MacBook Air models had the butterfly keyboard until 2018. Also, the following does not include the 2016 MacBook Air: Keyboard Service Program for MacBook, MacBook Air, and MacBook Pro - Apple Support

But the only way to know for sure what warranty exceptions Apple might make is to contact them. There is a Contact Support link at the top right of this page.