The model registry currently only supports the main database connection. The Contao framework supports opening additional database connections, however, you cannot make models use those.
More information: contao#6248
Non-admin users cannot copy or move content elements between different parent types, e.g. from an article to a news item or from a news item to an event. They can only copy or move the elements from e.g. one article to another article.
More information: contao#5234
The extension repository (ER2) does not handle dependencies correctly when an extension is upgraded. It does upgrade dependencies which are already installed, but fails to install new dependencies that have been added in the meantime.
More information: contao#3804