It's time for a proper API!
That's why I started to develop a phpBB API extension. For now it will be session-based, meaning it will use current user session as authentication mechanism, just like phpBB core does. As for now I plan to add support for all frontend entities (forums, topics, posts, members, ...), maybe later I will also add support for UCP, MCP and ACP.
What is the motivation? It breaks my heart to see all that styles that really change only few lines of CSS. Styling is bound to strict rules - exact files, only so much template variables, etc. That prevents us from creating really excellent styles.
With this extension installed, style authors can use frontend frameworks to paint whatever pages they want. Just start your application in page header, request data using newly exposed API and display them in any way you want! Questions about search engines receiving empty pages? PHP can render initial template.
Technically, phpBB API extension will expose a GraphQL server. With one request you can obtain all your data at once.