Added support for 1.21.3 (and with that 1.21.2) and 1.21.4
So, all 1.21 versions are now supported

Ideally there should be a single JAR. It's possible, but would be a hassle to maintain as some NMS classes have different packages even though the contents are exact same.

If you use an older version, you may be able to build the plugin yourself:
https://git.m724.eu/Minecon724/tweaks724
mvn package -Dproject.minecraft.version=1.21.4 -Dproject.nms.version=v1_21_R3