Note that these classes are immutable.

api/src/main/java/com/velocitypowered/api/proxy/messages/LegacyChannelIdentifier.java

@@ -7,9 +7,9 @@ import java.util.Objects;
 
 /**
  * Reperesents a legacy channel identifier (for Minecraft 1.12 and below). For modern 1.13 plugin messages, please see
- * {@link MinecraftChannelIdentifier}.
+ * {@link MinecraftChannelIdentifier}. This class is immutable and safe for multi-threaded use.
  */
-public class LegacyChannelIdentifier implements ChannelIdentifier {
+public final class LegacyChannelIdentifier implements ChannelIdentifier {
     private final String name;
 
     public LegacyChannelIdentifier(String name) {

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java

@@ -7,9 +7,9 @@ import java.util.Objects;
 import java.util.regex.Pattern;
 
 /**
- * Represents a Minecraft 1.13+ channel identifier.
+ * Represents a Minecraft 1.13+ channel identifier. This class is immutable and safe for multi-threaded use.
  */
-public class MinecraftChannelIdentifier implements ChannelIdentifier {
+public final class MinecraftChannelIdentifier implements ChannelIdentifier {
     private static final Pattern VALID_IDENTIFIER_REGEX = Pattern.compile("[a-z0-9\\-_]+");
 
     private final String namespace;