Salesforce Record-Triggered Flow can be used to Post Slack Message when a record is created or updated.

In the following Sample Flow, I am sending a Slack Message when a Case is Closed.

Prerequisites:

For Service Cloud for Slack setup for Swarming, check the following link:

Sample Salesforce Flow to create Swarm Request using Slack for Service

Sample Flow:

Start Element Configuration:

 Send Slack Message Element Configuration:

Output: