Linked Data Notifications (AKA LDN) is a W3C Recommendation developed in the Social Web Working Group to send RDF-based messages between clients and servers or between two servers, as well as how clients can retrieve these messages for reuse. It became a W3C Recommendation on 2nd May 2017.
rhiaro with sloph
Since 2016-06 Amy Guy accepts LDNs to a single Inbox advertised in the Accept Headers of sloph. There is currently no authentication or pre-emptive filtering, so all notifications go into a moderation queue. There has been no spam to date. Notifications which are interactions with specific posts using ActivityStreams 2.0 or SIOC vocabularies are displayed under the object post.
sloph also has code to send AS2 LDNs but nothing triggers it yet.
Kongaloosh with IndieAnndroid
Kongaloosh accepts LDNs but filters out any that don't appear to be sent from rhiaro.
dokieli sends LDNs for replies, likes, annotations and when you share an article with one of your contacts. dokieli also consumes AS2 notifications from an article's Inbox in order to display annotations and comments.