I like this less than simply raising the gold cost of upgrading units broadly.
The one major abuse I see with upgrading is pre-building the older, cheaper units, then mass-upgrading as soon as the new tech has unlocked, and deploying the recently upgraded force. This is most typically seen with an archer rush, where players pre-build slingers in anticipation of the calendar tech. If any of these proposals is actually about addressing some sort of perceived abuse of mechanics, and not just punishing players for upgrading units for the hell of it, then it should address this behaviour, and scaling the gold cost on unit levels in no way does that. If anything it seems to be a scalpel change to punish human player enjoyment while keeping an actual exploit intact.