The goal was never to feed beef. Fish can't digest animal fats, and the heart is the leanest muscle. I guess if it isn't, we know how the cow or goat died!
It was brilliant to realize this and improvise with it. As far as I can see, beef and goat heart recipes were the first home produced frozen foods, once refrigeration became easily accessible.
It's an enormous amount of wasted labour to make your own, compared to using more easily digested ingredients. It's the additions that make these recipes - the plant matter and colour enhancing elements. I took the recipe used by an old Discus breeder in our club, a man who died in his 90s in the 90s, and worked off of that with shrimp and fish bases.
I find it odd it's still used. It was a brilliant band aid solution, but better options are easy to find. We're very traditional in this hobby.
As for the question of natural - Discus need a lot of vegetable matter. But we aren't going to be able to feed them the plants they find in nature, so we use seaweed, spinach, carrots, peas, etc. And if the proteins come from heart muscle, it's no less natural than if it comes from North Atlantic shrimp or arctic fish.
We have to improvise. One of my favourite fish is my Epiplatys huberi, and in the wild they were eating a type of fire ant and the really small spiders that filled the bushes above them (and filled our nets when we caught the fish). Somehow, I can see those cultures going wrong.
Beefheart works. It's proven, and it doesn't seem to do any longterm damage. It was a great hack that's long since outlived its time.