but it's not wrong though. that's like saying you can't say 20% off and must provide the exact discount in whatever monetary unit you prefer.

it's not the same if you think about it a bit more

an equivalent monetary example would be describing something which costs 79 cents as costing "79% of a dollar"

sounds weird no?

Equally, we never say "50% of a m/s" instead of "0.5m/s"

And notations like 0.9c are also quite commonly used for velocities close to the speed of light

(the LHC tends to use TeV more often as a unit, which corresponds to a certain velocity for protons, but they mainly care about the energy)

But using percentages with the speed of light makes some sense because it marks one end of an absolute scale, there is no way to go 1.1c or 2.5c. So using a percentage emphasises how close we are to the maximum value.

But the same doesn't apply to AU.

0.9AU, 0.9C, 0.9 Parsecs, 0.9 miles, all make sense

half an AU, half the speed of light, half a parsec, half a mile, again fine

90% of an AU, 90% of the speed of light, 90% of a parsec, 90% of a mile. That sounds odd.