How easy is it to create a cipher which is very hard to break?

[-JKP-]

The term "steganography" goes back to Trithemius in the late 15th century. Do you have any examples of filler text being used in the early 15th century or before?

Steganography takes many forms. It can be filler text. It can be a diagram that hides letters within the way it is drawn. It can be a page of text with letters in the vertical direction forming a hidden message (similar to anacrostics) and numerous other variations.

The latter example existed in the Middle Ages, before the 15th century. The concept existed long before there was a term to describe it.

So steganography in a number of forms existed pre-15th century.

I do not know if the specific steganography scenario you describe (a high proportion of meaningless filler text) existed in the 15th century or earlier. I have not seen any examples so far but that doesn't necessarily mean it doesn't exist.