The implementation Add chat (!2002) · Merge requests · Tryton / Tryton · GitLab contains a minimal viable chat feature.
I plan to merge for the release 7.6 as an experimental feature (no model will have the chat activated by default in standard module).
For now it allows only user to chat together in “real time” (thanks to the bus) but there is not yet notification (aka Chat / notification modules) nor gateway to email.