Here is a rough example, assuming your feat's unique ID is "fShardWepProf".
Post-Level 10000
Code:
~Do nothing if we have the correct proficiency
doneif (hero.childlives[fShardWepProf] <> 0)
~Otherwise travel to our parent weapon and make it not proficient
perform container.parent.delete[Helper.Proficient]
Instead of childlives, you could add a script to the feat you created to assign a Custom tag you define to the hero, and check for that in the material script. Either way would work I think.
Usual disclaimer about code being untested and may need tweaking.