The Restlet website defines the framework as the Leading Java Framework for building APIs (if I remember that correctly) and for a time Restlet was very much active in the community. After the acquisition of Talend, it seems the direction of Restlet has changed.
Furthermore, Restlet compared to other Java frameworks seems to be lacking modern features that an adequate number of applications (if not most) needs, WebSocket, non-blocking I/O, and reactive features missing in the framework and it seems it is quite lagging compared to other frameworks. Some important features that are either not implemented for quite a long time or just unplanned for the core developers.
The question now, in terms of programming, is it still reasonable (today) to invest in developing applications using the Restlet framework or it is much better of picking up frameworks like Vert.x or Quarkus (JAX-RS) or Spring instead?
I understand the concerns with lower core team reactivity. We have actually invested a significant amount of resources at the end of 2019 and early in 2020 to ensure a smooth migration of Restlet Framework to Talend's infrastructure (incl. full Maven repository migration with minimal visible impact, plus website migration to GitHub Pages) and released:
Version 2.4.3 with fixes: https://restlet.talend.com/documentation/2.4/changelog
Version 2.5 M1 with important clean-up and scope reduction to facilitate ongoing maintenance and evolution: https://restlet.talend.com/documentation/2.5/changelog
We are currently assessing how we will drive the framework moving forward, beyond 2.5, either with 2.x or 3.x. Some of the technologies we are considering are cloud-native (service mesh, serverless, OAS 3 and alike).