There are multiple large Rails services (I'd guess GitHub included) that internally have a majority of contributors in favor of an API first approach, but it's not mandated absolutely.
Shopify has a component boundary interface in Ruby that can be reflected into with GraphQL, and a lot of features are built for GraphQL first anyway. First party client side apps demand it. A lot of internal services use GraphQL to talk server-server as well.
However, there's still a good chunk of monolithic logic left that hasn't been refactored yet. Refactoring efforts are mostly JIT when demanded.
Shopify has a component boundary interface in Ruby that can be reflected into with GraphQL, and a lot of features are built for GraphQL first anyway. First party client side apps demand it. A lot of internal services use GraphQL to talk server-server as well.
However, there's still a good chunk of monolithic logic left that hasn't been refactored yet. Refactoring efforts are mostly JIT when demanded.