Switch Online's "Mature" Nintendo 64 App Is Now Available In The West
What specific titles are included in the newly launched "mature" rated Nintendo 64 app for Switch Online users in the West?

The newly launched "mature" rated Nintendo 64 app for Switch Online users in the West includes Turok: Dinosaur Hunter and Perfect Dark. These titles were announced alongside the app's release. It is worth noting that GoldenEye 007 and Jet Force Gemini, which were initially released in Japan, are not included in the Western version of the app2.
When did the "mature" rated Nintendo 64 app originally debut in Japan, and which games were released alongside it?

The "mature" rated Nintendo 64 app originally debuted in Japan in November 2023, alongside the release of GoldenEye 007 and Jet Force Gemini3.
Can users integrate games from the new "mature" rated app with those from the previous N64 releases on their Switch Online interface?

No, users cannot integrate games from the new "mature" rated app with those from the previous N64 releases on their Switch Online interface. The new app is a separate tile on the Home menu, and there is currently no way to merge the games. Both apps need to be accessed individually, and an Expansion Pack subscription is required to play the games from the "mature" app on the Switch.