Network filters (e.g., Securly, GoGuardian, Lightspeed) now use:

The days of Adobe Flash (SWF files) are over, which killed a lot of old unblocked sites. However, and WebAssembly have resurrected the genre. Games today run faster, look better, and leave no history if played in incognito mode.

9 Game Sites Not Blocked by Schools [2025 Updated] - AirDroid

These games require only a pen and a piece of paper, making them impossible for a firewall to block.