This is a request coming from a colleague, so I am not fully aware of the exact use case here,
It seems that recently the ability to watch/subscribe to a project was added (T4967). In addition, membership to a project currently automatically implies subscription. It could be useful to separate these relationships completely (that is, membership and subscription) such that members of a project can unsubscribe from a project.
The issue we are having is that we have a fairly large project which has a half-dozen members. The members of this project are receiving a lot of email which is largely irrelevant to them.