Except the difference between QNH and STD could be huge. Your Cessna could be flying at 200 ft above ground in your scenario.
All primary altimetry would be based on GPS. As back-up pressure altimeters would be set to local QNH (or 'altimeter' in the US). Light aircraft (the Cub or C152) would use local QNH if not equipped with GPS altimeters. 'Standard' setting (1013 hpa) would disappear, along with Transition Altitudes (which are another potential source of setting errors).
ICAO resolution 23/2020: 'No non-Russian registered aircraft will operate within Russian airspace unless their crews are permitted to use absolute altimetry (derived from GPS signals or local sea level air pressure) at all levels. Altimeter scales will be graduated in Imperial feet but indications (divided by a factor of 100) will be referred to as 'flight levels' at all levels.'