Is there a plan for the development?

Yes. For version 2019, a large part of the code-base was rewritten to allow for more sophisticated code-insight like package-wide completion and navigation. A good part of the development time is spent on implementing new features and fixing bugs. The goal for the near future is to provide better support for Paclet-based packages. However, often the direction of development is influenced by users with good arguments for a specific feature. If you are interested in discussions, please visit the #jetbrains-plugin channel on Slack and report bugs on GitHub.

Can I deploy my package?

Basically yes. Deploying an old-style package is not more than creating a .zip file from your package sources that users can extract into their Applications directory. You can create an Artifact in the IDE that does this. For Paclet-based packages, the best way to deploy code is to run PackPaclet on the directory which contains the PacletInfo.m file.

Can I run or debug my code inside the IDE or build documentation?

Currently, not. Both actions require a Mathematica kernel, and while the communication with the kernel is well-documented, the way the Wolfram Workbench interacts with the Mathematica front-end is not. That means, information about debugging code or building documentation is scarce and hidden in the implementation of Wolfram Workbench. Especially, building package documentation has proven to be extremely fragile and does not work well when supporting several Mathematica versions. Hopes are that in the distant future Wolfram will provide an official and stable way how this is done so that it can be implemented for the Wolfram Language Plugin as well.