Of the email notifications that are sent for talk page events, what proportion of them are opened?

Most email clients prevent this being tracked even if we tried (and I don't think we do try)

Of the email notifications that are sent for talk page events, what proportion of the links they contain are opened?

The links in the email are the same as the links in the notification popup. In theory one could guess based on the referrer (assuming it's a web client), but again I doubt we do this for privacy reasons.

@ppelberg It looks like this task isn't high priority; can we move it to our backlog?

In T259184#6522409, @kzimmerman wrote:

@ppelberg It looks like this task isn't high priority; can we move it to our backlog?

Yes. Thank you for checking, @kzimmerman.