This Zulip feature:
https://zulip.com/help/read-receipts

The API endpoint for this feature is:
https://zulip.com/api/get-read-receipts
Unlike most Zulip features, this data isn't controlled by the event system; we just fetch it separately from the server.

See the zulip-mobile implementation for a model to work from, in src/message/ReadReceiptsScreen.js. We can use similar logic: in particular, the data won't live on PerAccountStore at all, but only in the state of the StatefulWidget for the read-receipts screen.