If you're interested in knowing more about the science behind the protocol, a very good paper is "Critical Flutter Speed of Sailplanes Calculated
for High Altitude: Examples of computation" by Wojciech Celcstyn, 1993, a featured article in Technical Soaring: https://journals.sfu.ca/ts/index.php...e/view/216/200.

Fig 1 top half shows shaded flutter region for one oscillation mode and the dashed line how one would enter it flying fast enough IAS at 2000m (EAS and IAS are the same for glider speeds). If you flew at higher altitudes you would enter it at a lower speed, given by the parabolic looking curve going upward to the left, which gives the true altitude change of IAS-VNE flutter. The real change is NOT given by constant TAS, but the TAS line is shown for comparison. It's simply a convenient line that has enough slope so it will keep you out of danger in most cases.

The other figures show other modes. If you read the conclusion, it's clear that the standard (German and everyone else I guess) VNE protocol is not based on knowing where flutter is, either by testing or calculation. It's based on testing to a certain altitude and finding no flutter. Then IAS-VNE is chosen to drop off as TAS is held constant to stay away from unknown flutter at higher altitudes.