I’m not sure this is a good idea. Notifications are received very often, perhaps every day. For those, a short slug works well.
The idea with a digest is it’s received only occasionally, and reminding people to come back to the site. Having the short slug version may not make it obvious from the subject line what the email is about or who/where it’s from. Especially now that the subject is shortened to just “Digest” with no date, the subject line shouldn’t be too long in most situations.
I for one want the full site title in the “come back” digest, just not the hundreds-per-day notifications.