The only way to accurately track this worldwide is brakes released / locked. It might be possible to have an agent watch to see when an aircraft pushes or actually pulls into the gate, but everything is subjective.

The EU is the outrider. Alone among all nations (or groups of them), courts in Germany have determined that for purposes of calculating EC 261/2004 compensation, the arrival time is calculated by the time when the cabin door is opened for passenger deplaning.

As with EC 261/2004 itself, nobody else followed suit and likely nobody ever will. You can protect yourself by flying only LH or carriers departing German airports.