The upstream minecraft server is a JAR that was run through a Java bytecode obfuscator, which mostly works by renaming all functions and fields with names like that. I'm not aware of any open-source server built with such unreadable naming from the start, but there are definitely _decompiled_ versions floating around. Since the original JAR was obfuscated, these decompiled versions are only partially deobfuscated, if at all, hence the unreadable names.