The composer-merge-plugin project is currently managed exclusively as a GitHub project. This hosting choice was made as an experiment to see if it would attract more outside development interest in a software component that is critical for MediaWiki 3rd party use but also generally useful for the larger Composer and PHP developer communities.
By and large this experiment has been successful. The feature set of the plugin has grown in response to feature requests and code submissions from outside the typical MediaWiki contributor base. It has also been useful for allowing easy communication with the related upstream Composer project's users and maintainers who are typically used to GitHub interactions.
Where it has really fallen short is in the usefulness of GitHub's code review process. GitHub has no support for dependent change sets (pull requests) and seeing the history of changes that have been developed using a rebase-centric workflow is very difficult. The pull request model also leaves the project's commit history littered with merge commits.
I'm interested in being a test project for differential code review. I'm especially interested in helping figure out if we can build a workflow for code review similar to the system used by HHVM. In their process, patches can be proposed as a GitHub pull request but the review takes place in their Phabricator instance. Their bot integration even allows updating the Differential review by force pushing or adding a new commit to the GitHub PR.
Composer-merge-plugin is a self-contained and low volume project so it seems like a nice candidate for experimenting with new workflows that may ultimately be useful to a wider range of Wikimedia projects.