This is anecdotal, and no idea how real it is, but my dad flew reconnaissance for the USAF in the 70s and claimed to have the capability to read license plates way back then from very high altitude (no idea what qualified as very high altitude, but assumed they were not easily detected from the ground).
Let's do some napkin math.
A quick websearch suggests spy sats orbit around 200-800km, while U2 flies at 20km.
Let's say you can use the same camera system on both. Then 10cm resolution at 200km orbit means 1cm at 20km, which would be enough to read a license plate.
