While

https://github.com/redwoodjs/redwood/blob/895c2d09552e30138a9da09d447caea353f2a5ca/packages/api/src/functions/graphql.ts#L90

does turn off the playground if in dev mode, because it relies on process.env.NODE_ENV

process.env.NODE_ENV !== 'production'

and this may not be set in Netlify, the GraphQL playground could be exposed.

export const createGraphQLHandler = ({
  context,
  getCurrentUser,
  onException,
  cors,
  onHealthCheck,
  ...options
}: GraphQLHandlerOptions = {}) => {
  const isDevEnv = process.env.NODE_ENV !== 'production'
  const handler = new ApolloServer({
    // Turn off playground, introspection and debug in production.
    debug: isDevEnv,
    introspection: isDevEnv,
    playground: isDevEnv,

How to fix?

• make sure process.env.NODE_ENV is set? (edit: nope ...)
• be explicit and have GRAPHQL_PLAYGROUND_ENABLED env set only in dev and check that value for enabling the playground

Notes:

https://community.netlify.com/t/node-env-is-undefined-in-lambda-functions-no-matter-what-i-do/20667/2

Issue at hand is the Netlify Build vs Runtime environment. You will have to explicitly set NODE_ENV as a Netlify environment variable in their admin UI.