We have a 50 year old English walnut tree and wanted to landscape around it with stone and fill in with dirt and plant flowers on top, then I read that it will kill some trees.
Is this true?
Yes it is. If your tree has buttress roots that stick out above the soil, these should not be covered in any way, and the soil level around the trunk should not be raised even if buttress roots are not present. If you want to landscape with stone nearby, care should be taken not to destroy any roots that belong to the tree which are beneath soil level. Any of these changes or damage to the roots can result, ultimately, in the death of your tree.
It is also unwise to plant under the canopy of a walnut tree - they produce a substance called juglone which can be toxic to other plants, see information here https://extension.psu.edu/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants