Webhook message type `Delayed`

There’s a new webhook message type called Delayed - I couldn’t find any information on when / why these messages get fired by searching the documentation / changelogs. Is it documented somewhere?

Add new Delay log record type · KumoCorp/kumomta@cf6f354 · GitHub is the commit that added it. You can see that it also updated the log_record docs, and missed updating the changelog to point to it