The 1.21 branch is open, from 1.21.4 up.

Nothing in the engine changed. What changed is what it is built against: 1.0.0 carried api-version: 26.1.2, copied from the hosting image, and Paper refused it on anything older with Unsupported API version. That was never a decision, so it is gone.

• Java 21 is enough now, where 1.0.0 wanted Java 25. The 1.21 branch only requires Java 21, and a jar compiled for Java 25 was refused by the JVM itself, before Paper ever read the api-version.
• The floor is 1.21.4, and it is measured, not guessed. Below it, two catalogue actions have nothing to bind to: player.give_effect and player.heal. api-version is 1.21.4, so Paper refuses the install instead of letting a rule throw mid-game.
• The grammar does not move. specVersion is still 1: a spec written for 1.0.0 runs as it is. 9 triggers, 8 conditions, 14 actions, and no network calls, same as before.

Started on a running server, console read, one spec firing all nine triggers: 1.21.4, 1.21.8, 1.21.11, 26.1.2 and 26.2. Link-checked against all eight declared versions. 1.21.5 and 1.21.9 are left out because PaperMC publishes no stable build for them, not because anything fails there.