I just had a look. I can absolutely understand parent. I’d want an option to include read messages in the inbox, not avoid marking as read. I want a history of stuff in my inbox, the same way discord, my RSS reader, and my email client work. All those have a read and unread state, but I can still see the read ones.

You may be looking for the recent view (https://zulip.com/help/recent-conversations) which you can also set to be the default/home view for your account.

(And administrators can set it to be the default for new users in their organization, if the way your organization communicates is such that it's a better default than inbox).