Are non-Minecraft projects allowed?

Short Answer: Yes

Long Answer: CodeMC accepts any Java (or Java-related) project that shows thorough quality and usefulness for the overall programming community, be it a library, software, or even a game. If you're using Kotlin or Scala, your project should compile to the JVM while continuing to provide services intended for the community. What matters is, that the project is following our Usage Guidelines to remain on our Services.