you don't need to use a custom property, you can assign a value to it, and you can get this property in html

mm-hmm, got it. Setting column-key is indeed a method, but considering its uniqueness, it is not appropriate if your custom properties may be duplicated. On the contrary, if the custom attribute is id and unique, you might as well use it boldly.
if you must customize an attribute that does not affect other functions, you can use render-header to render the header, and add the attribute to column during render:

<el-table-column :render-header="renderHeader"></el-table-column>
...
renderHeader(h, { column, $index }) {
  column.test = 'test'
  console.log(column) // 
  return (
    <span></span>
  )
}
...
// :render`ElTable`