But this is not such a big deal.
I made an open-source lightweight daemon in Go that fills that gap. All it does is to provide the means to connect to popular messaging systems like Slack, Discord, WhatsApp, Telegram, etc. and expose this all through the CLI.
The project is hosted here: https://github.com/pantalk/pantalk
My personal realisation recently has been that the unix way is the best way. We just need to go back creating daemons and lightweight composable CLIs and let agents do their thing. They are increasing being trained to operate the command-line and they are getting pretty good at it.