Well ... it's been some hours since 4.21.0 xD
Anyway, this very small update fixes the usage of underscore in slash commands (+ gorups and sub-commands). If you do not use any `_` in your commands, then you don't really need to update for now.
Developer information
Projects on Modrinth are automatically available through a Maven repository for use with JVM build tools such as Gradle.
maven { name = "Modrinth"; url = "https://api.modrinth.com/maven" }