Aurora still supports the legacy REST APIs, although the url path needs to be updated to append /t5/s to hit those legacy REST endpoints. I assume your developers aren't directly using LiQL or SQL rather that they are using REST endpoints where they pass LiQL to execute a query and get a response. While the REST APIs are still available today with that URL change, they will be deprecated in a future release with plenty of prior notice given.
GraphQL is a query language but it still works through basic HTTP commands (post), the difference is you are passing along GraphQL queries in the request, often to a single endpoint, to request exactly the data you want returned. It's a more standardize structure to our proprietary LiQL queries. Outside of some helper libraries that making caching and state management easier, if any 3rd party app is making REST API calls, the transition to GraphQL API can use similar web/http requests. This can be experimented with basic web tools like Postman or even javascript fetch commands, curl etc.
