Google is in the process to modify its notification panel to the Google Phone app. Currently when we are in call on Android , swiping down to see your in-call notification screen reveals nothing more than an option to end the call.
However, spotted in the AOSP commit (via XDA), Google seems to be working to bring a new actions to its notification drawer, that is enable or disable the speakerphone in the notification drawer instead of opening/ moving to phone app to switch on/off the speakerphone. Till now there is no idea on the time line as when it will added to the Phone app, and expected to happen soon in the future. Google might add some additional features here as well.