`GatewayService` implements logic for delaying state recovery in the cluster until enough data nodes have joined and/or some timeout has occurred. The implementation is pretty old and doesn't make use of much modern infrastructure for doing this kind of thing, making it pretty hard to follow and to improve (see e.g. #86588). We should clean it up and simplify it.
test/framework/src/main/java/org/elasticsearch/common/util/concurrent/DeterministicTaskQueue.java
public boolean isShutdown() {
                    return false;
                }
                @Override
                public boolean isTerminated() {
                    throw new UnsupportedOperationException();
                }
                @Override
                public boolean awaitTermination(long timeout, TimeUnit unit) {
                    throw new UnsupportedOperationException();
                }
                @Override
                public <T> Future<T> submit(Callable<T> task) {
                    throw new UnsupportedOperationException();
                }
                @Override
                public <T> Future<T> submit(Runnable task, T result1) {
                    throw new UnsupportedOperationException();
                }
                @Override
                public Future<?> submit(Runnable task) {
                    throw new UnsupportedOperationException();
                }
                @Override
                public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
                    throw new UnsupportedOperationException();
                }
                @Override
                public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) {
                    throw new UnsupportedOperationException();