ViaVersion requires java 17+ in general. Here are some things to note:

• See https://docs.papermc.io/misc/java-install on how to update your Java version
• Even Minecraft 1.8 is mostly able to run on Java 17, although for a better experience you should use a fork that fixes smaller issues
• Some older versions require the Paper.IgnoreJavaVersion system property to be set (-DPaper.IgnoreJavaVersion=true as a startup flag), but run perfectly fine using Java 17
• If all else fails, use the Java 8 compatibility builds we provide at risk of not receiving support for issues it could cause - you should not make yourself depend on these too much, they will generally only be updated on releases!
  • ViaVersion - https://ci.viaversion.com/job/ViaVersion-Java8/
  • ViaBackwards - https://ci.viaversion.com/view/ViaBackwards/job/ViaBackwards-Java8/
  • ViaRewind - https://ci.viaversion.com/view/ViaRewind/job/ViaRewind-Java8/

At some point in the future, ViaVersion will require at least Java 21 to be run. Please make sure your server can run on Java 21+.