latest version of home assistant
Can you note your HomeAssistant version and Slack integration version? Saying ‘latest’ version of Home Assistant is ambiguous. And makes it more difficult for others to help and for other to make use of this knowledge in the future.
It’s not a direct fix but you could try using the notify.notify service. From the docs:
notify.notifyis shorthand for the first notify service the system can find and might therefore not be working as intended.
Or alternatively use Visual Studio Code to do a Find & Replace on all your automation that have the old notify.slack.