Did those people sign a legally-binding end-user license agreement?
Because what I see is essentially "they're storing stolen property" but the burden of proof is on the author to prove it was, indeed, stolen.
I imagine the bar for that is pretty high otherwise anyone could weaponize DMCA to target their competitors' repositories.
People are absolutely abusing the DMCA process.
https://reclaimthenet.org/kiwi-farms-dmca-subpoena-anonymous...
You seem to be thinking about "adblocker running in browser", since you brought up EULAs.
That's not what OP alleges - they are saying people are redistributing modified versions of OP's copyrighted code. DMCA is an appropriate measure in such a situation, but it's unclear why OP's DMCA takedown was rejected by GitHub. Without more detail, it's hard to comment further