Clay is unlike any of the granular materials that are found in sub-soils. Clays have "plasticity" and can typically be molded into simple shapes.

You can actually cut a vertical wall in moist clay. If the clay has good cohesion, the wall will not slough into the cut until it starts to dehydrate - which causes the clay to shrink and crack.

However, that is not your only consideration. Pond slopes are almost always subject to wave erosion. This force is not very strong in a small pond, but it is relentless. A clay slope that gets slapped with enough tiny waves will eventually start to erode - and the likelihood of erosion increases with the steepness of the slope.

There is a picture in one of the Pond Boss books where they cut some vertical clay walls in the deeper sections of the pond. I thought that might be an interesting option in my clay-rich soils.

I believe I even posed the question on the forum about the long-term sustainability of a vertical clay wall at depth. It got some responses, but no one was definitive either way that they would work OR that they were sure to fail.

In my non-expert opinion, I would probably try to keep the shore slopes at about 3:1. However, if you are excavating in pure clay AND your pond will not be subject to significant water level changes, then you could try some steep cuts at depth.

If you have cobble size (or larger) rocks on your property, you could probably even cut a vertical wall and face it with stone. That might be an excellent fish-concentrating feature that you could create in a few places during the excavation phase.

Since it will not be visible from the land, you could even go ugly and dump concrete construction rubble against the wall. Also, if you are clearing some good wood like hedge trees, you could even crib a steep wall.

Good luck on your pond build!