Outlook ItemSend event as a standard Office.EventType usable in Taskpane code

Good luck. MS devs have become the new overly opinionated Apple devs.

You can still achieve what you want by setting session data in the taskpane every second and then checking it in a Smart Alert to see if more than a second has elapsed. This will tell if you if the Taskpane is open.

By design you need to duplicate any code you wish to execute in the Smart Alert. And just to save you a lot of headache, the Smart Alert runs with the same modern javascript engine as Outlook.com, but the Outlook native version uses some stripped down JS engine that doesn't support any modern JS features and will be a huge pain to develop against.

From Taskpane:

Office.context.mailbox.item?.sessionData.setAsync("taskPaneTimestamp", new Date().toISOString());