Zulip servers (at API feature level 174) support "direct" as the preferred message type for direct/private messages when sending messages and sending typing notifications; see zulip/zulip#25154.

For servers at this feature level, when sending direct messages (POST /messages), the mobile app should switch to using the new preferred "direct" value for direct messages.

For typing notifications, the default value was switched from "private" to "direct", so no changes need to be made by clients if the default is being used for direct/private message typing notifications.

See this #api design conversation for more context and discussion on the type parameter.