You have options that are both legal and safer. Here is how to get your Binding of Isaac fix without resorting to shady unblocked sites.
Legitimate players spend 100+ hours unlocking "The Lost," "The Forgotten," or "Godhead." Cracked versions often break save persistence, meaning you lose progress every time you close the browser. The Binding Of Isaac Rebirth Full Game Unblocked
If you simply want the feel of Isaac without risk, the original 2011 Flash game has a demo on Newgrounds (still accessible on many networks). It is not Rebirth, but it is free, runs in a browser, and introduces the core loop. You have options that are both legal and safer