Have we have seen Goku back out by fear and shock before?

[King13s]

Short answer, no.

Long answer, Goku doesn't get scared of overwhelming power. Any opponent of a strength that has others shaking, he anticipates. It is his nature to be excited by that sort of situation. He's a fighting adrenaline junky. Nobody has ever rattled Goku to the point of backing down. Brolly made him get some space, and he had no idea how to fight the monster, but he was just trying to figure things out.

If Goku had his way he'd never even leave a fight. Losing or not

[TheShadowEmperor8055]

He's been afraid before, yes.

But to be so afraid to literally power down from his transformation temporarily to take a few steps back in sheer awe of his opponent? No, we haven't seen that before.

[Kataphrut]

I feel like Super Buu was the only one Goku ever backed down from to the point of refusing to fight. Raditz, Freeza, Hit and Jiren have all made him tremble, but he still tried to keep fighting all the same. Buu absorbing Gohan was the one time he was basically petrified and had no choice but to skedaddle.