I expect copyofhey to be it's own variable with it's memory space... but using the remove_hash_key filter it just removes from all everything ... I expect copyofhey not to be touched when:

{%- parse_json hey -%}
{"test": "hello", "test2": "goodbye"}
{%- endparse_json -%}

{%- assign copyofhey = hey -%}

{%- assign nothingofinterest = hey | remove_hash_key: "test2" -%}

{{copyofhey}}

{{hey}}

Yet the output is:

{"test":"hello"}

{"test":"hello"}