Valid user mention in Slack notifications
At the moment Slack notifications do not properly mention users.
The idea is to add "Slack ID" field (similar to existing "Pager address") in users configuration and later on pass new variable "CONTACTSLACK" with this ID to notification script.
Please check this discussion on forum for details: https://forum.checkmk.com/t/how-to-tag-users-in-slack-notifications/43476
My current workaround is described in the same topic here: https://forum.checkmk.com/t/how-to-tag-users-in-slack-notifications/43476/16