Where can I find Babelfish release notes or recent features?

Detailed release notes for Amazon Aurora, including Babelfish changes, can be found here: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraPostgreSQLReleaseNotes/AuroraPostgreSQL.Updates.html

A simple table of features by Babelfish version can be found here: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/babelfish-compatibility.supported-functionality-table.html

If you'd like to keep up-to-date with new Babelfish features, you can subscribe to the AWS What's New. There is an RSS feed: https://aws.amazon.com/new/

Finally- Babelfish is an open source project. If you're looking for release notes from the open source project, Babelfish for PostgreSQL, release notes can be found at the project website.