Detecting FeatherMC

Most plugins never need to know. If you want to show it in a debug command or enable something only on FeatherMC, check the brand name. The brand ID is deliberately still Paper’s.

import io.papermc.paper.ServerBuildInfo;

public static boolean isFeatherMC() {
    return "FeatherMC".equals(ServerBuildInfo.buildInfo().brandName());
}

// Bukkit.getName()                      -> "FeatherMC"
// ServerBuildInfo.buildInfo().brandId() -> papermc:paper

Don’t use this to work around Paper behaviour. If something behaves differently on FeatherMC than on Paper in a way your plugin can see, that is a FeatherMC bug and worth reporting.