Reminders represent an action to be done to a page. So a generic page icon could work well in this case. If we want to emphasise their time nature (i.e., this is shown because you scheduled it in the past), we can combine it with the clock icon.

The version with the clock works for me.

The design part of this task is complete. Someone just needs to implement it, but I don't think that's a collab project, so I'm moving this to the Triage board, "external" for Collab team.