The MediaTrackSettings dictionary's echoCancellation property is a Boolean value whose value indicates whether or not echo cancellation is enabled on an audio track. This lets you determine what value was selected to comply with your specified constraints for this property's value as described in the MediaTrackConstraints.echoCancellation property you provided when calling either getUserMedia() or MediaStreamTrack.applyConstraints().
Echo cancellation is a feature which attempts to prevent echo effects on a two-way audio connection by attempting to reduce or eliminate crosstalk between the user's output device and their input device. For example, it might apply a filter that negates the sound being produced on the speakers from being included in the input track generated from the microphone.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.echoCancellation as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.
Because RTP doesn't include this information, tracks associated with a WebRTC RTCPeerConnection will never include this property.