I hope you all do well with this!
Isn’t that what subject lines and mail archives are useful for?
Nobody has put together a simple solution in the mail domain. Maybe there's a market for it - turns out someone might pay for it if it's better than the slack experience.
There's always newsgroups, which use NNTP (which isn't that dissimilar to email). They have existing groups in a hierarchy that can be subscribed to. It doesn't really give users the ability to create new newsgroups in the fly though.
This would also be acceptable IMO. Any medium that enables asynchronous long-form written communication has advantages that are fundamentally impossible for any synchronous chat medium to match.
It turns out that listservs have successfully done this for decades. If there was even a fraction of the effort invested in this communication style as is invested in reinventing IRC with emoji and reaction gifs, I'm sure it would be even better.
With email, how can I subscribe to conversations I'm interested in? Mailing lists are nice, but we have no real way to make mailing lists that start and stop on the fly like a chat channel in a way that is searchable to the entire organization during or after the fact.
What email list system allows for quick creation and the ability for everyone in an organization to know that a conversation is happening and how to join with the ability to get all the context necessary in a chronological order? (Reading threads can get really repetitive in a mailing list, especially when trying to get up to speed on an issue.)
You get to all the requirements, and you end up with a discussion board instead or a new product that is to email lists what slack is to chat.
Email is great for some things, but there's a reason people use slack instead even when they already have email.