Matrix.org provides a public free Slack bridge, which is free to use forever but comes with some limitations:
This guide explains how to use the free Slack bridge from the Matrix.org Integration Manager to integrate your Matrix room with a Slack room.
Note that EMS offers a paid Slack bridge with more features.
It requires your homeserver to be able to federate with Matrix.org.
An EMS server is not required.
Create a new room in Matrix, with encryption off
Click Room Info in the top right corner of the room
Click Add widgets, bridges & bots
Add widgets, bridges & bots
Choose Slack from the list of available bridges and integrations
Click Add Bridge
NOTE if you have purchased your Slack bridge from EMS: Ensure it says Slack integration on <your ems domain> here.
Slack integration on <your ems domain>
Click Add to Slack
Add to Slack
Enter your Slack workspace URL, and click Continue
Enter your Slack email address and password, then click Sign in
Close the Slack tab and return to Element
Click List channels
Click the Slack channel you want to bridge to the Matrix room
Slack is now added to the Matrix room
Go to the channel you selected on Slack, and add the Element App via the Integrations tab in the members sidebar
The Matrix room and Slack channel are now bridged