According to the latest commits in AOSP (Android Open Source Project), Google is adding In-Built/Native Call Recording support to Android 9.0. While recording the call, the device will produce a tone after every 15 seconds and the sound will be heard by both the parties on the call, allowing the other party to confirm the call is being recorded.
However, operators will be able to choose whether to enable or disable the call recording functionality as it is illegal to record calls in few countries.
Just like every year, Google is expected to reveal the next version of Android OS, "Android P" followed by Multiple Developer Previews and a Full launch. This latest Android OS will reportedly come with new "smart features." Stay tuned to our Facebook and Twitter page to get the first News.
As clearified by Mishaal Rahman (XDA) on Reddit:
The commits do not show evidence of Android P implementing native call recording or an API for third-party call recording.
I have been following these commits since before they were merged, and the gist of why this is inaccurate is because the mention of recording apps is just so Telecom can warn that a call may be recorded. It does this by playing an outgoing 1400Hz tone every 15 seconds while a call is being recorded.
The specific carrier that asked for this is Vodafone in Germany, which you can verify by looking up the MCC/MNC combination mentioned in the file name of one of the comments.