Concrete, rock.

A long time ago when I was a little boy, my uncle had a farm pond.
When you walked in the pond you would Sink knee deept in red mud.
He was talking about something to put into the water that would settle to the bottom of the pond and cause the mud to harden . It may have been against the law. He never tried it. Anybody have any ideal what he was talking about?

I think what you may be referring to is bentonite clay. Many pond owners who have leaky ponds will cast this into the pond to harden the bottoms and fill leaks. If you really want a hard bottom to make swimming areas hard and walkable, I suggest spraying gunite concrete. Its the same thing they use in pools and the pond does not have to be drained.

There is nothing that can cause 1-2' of muck to harden underwater and still be a pond. Legal or otherwise.

A pond can be filled with concrete, but it wouldn't be a pond, it would be a block of concrete.

You can dump rocks into the pond and they will sink into the muck and more rocks are added until the bottom is "hard". But unless you start with a really deep pond you won't have much pond left.

When a mud pond is first build it is pretty hard, clay. Walking on it is like walking on concrete. After water is added the bottom does get a little muddy, you'd sink in maybe 1/2" or so, depends on soil. Over the years dead plant material collects in the pond and decompose, just like in a compost pile, which creates soil, which in a pond is muck. Builds up and in water is fairly weightless so doesn't pack down like in a compost pile and you have muck. And you sink to your knee in it.

Mud ponds are sometimes drained and re-scraped to remove the muck, and the process starts over. If this isn't done the pond will continue to fill with muck, pond becomes a swamp which becomes a bog which becomes dry land.

After being re-scraped concrete can be laid down. But muck will still continue to collect so you wouldn't gain much unless you had a way to kept removing the muck.