These aren't designed to be extended in any way.

proxy/src/main/java/com/velocitypowered/proxy/util/collect/CappedSet.java

@@ -9,7 +9,7 @@ import java.util.Set;
 /**
  * An unsynchronized collection that puts an upper bound on the size of the collection.
  */
-public class CappedSet<T> extends ForwardingSet<T> {
+public final class CappedSet<T> extends ForwardingSet<T> {
 
   private final Set<T> delegate;
   private final int upperSize;

proxy/src/main/java/com/velocitypowered/proxy/util/collect/Enum2IntMap.java

@@ -6,7 +6,7 @@ import java.util.EnumSet;
  * An immutable map of {@link Enum} entries to {@code int}s.
  * @param <E> the enum type
  */
-public class Enum2IntMap<E extends Enum<E>> {
+public final class Enum2IntMap<E extends Enum<E>> {
   private final int[] mappings;
 
   private Enum2IntMap(int[] mappings) {