Talking about OJS3: I believe it should be possible to use email templates also in discussions.
The editorial workflow changed a bit from OJS2 and now some templates simply can not be used anymore, although they exist. As far as I understand the email templates are more or less available only at assigning someone to the editorial process.
One example: in the Production phase an (graphical) editor would like to ask the author for proofreading. That is something normal and there is the template for that. How to do it in the OJS3? We are using discussions for that. So the editor opens a discussion asking the author for the proofreading. But, there is no way to use the ‘authors proofreading’ email for that. While that template can be used using assign feature. Since the author is already assigned to the submission, can not be again.
I believe the root cause of the problem is that in OJS2 the editorial process was more strict and for each step there was and email template defined. In OJS3 there are discussions available (very welcome feature) and some steps from the OJS2 strict workflow are now in the OJS3 to be accomplished using discussions. For such purposes the email templates should be available in the discussions as well, at least when creating new discussion.