How come installSubscriptionHandlers method is not part of apollo-server-express source code and this method is mentioned here in apollo-server docs
const http = require('http');
const { ApolloServer } = require('apollo-server-express');
const express = require('express');
const app = express();
const server = new ApolloServer({ typeDefs, resolvers });
server.applyMiddleware({app})
const httpServer = http.createServer(app);
server.installSubscriptionHandlers(httpServer);
This code works and doesn't show any error despite installSubscriptionHandlers being not a method exported from any module in the package source code.