How does it work for you?
As the popularity of other instant messaging platforms ebb and flow, email has remained one of the few ubiquitous methods for discourse, messaging, and notification.
In addition to its ubiquity, I like that email enables slow thinking / maintaining focus and doesn't require an always-on internet connection.
I also like that it's decentralized; though some of the bigger players have certainly gained enough users to control the ecosystem a bit, Fastmail, ProtonMail, Posteo, Gandi, and others, as well as self-hosting are all available.
In terms of client software, mu4e has been absolutely fantastic for searching, and indexing (via mu), threading, marking, replies, etc. making email work even better. mutt was also a favorite MUA for many years before my journey into Emacs.
For responding to SMS messages via email, voip.ms gets the job done as a SMS/email gateway, helping to consolidate the majority of my asynchronous messages into the same platform.
I could probably tie system notifications into Discord but I prefer email, as I control both the client and servers. I am probably just used to having control but I prefer to keep it that way. It would be interesting if some day Discord supported federation and self hosted leaf-node servers with granular control over what channels are replicated.
It is widespread, whereas Discord, Zoom, Skype, etc. are not available in lots of businesses. It's no good having Discord if the person you want to communicate with doesn't have it too.
It is also good enough and easy enough for non-techo users.