We have made a new integration with Slack. Now, using the old integration, you can only create your application (bot) that clients can communicate with. If you want to process messages from Slack in Usedesk by support forces, and you don't need a bot, the new integration is fine.
Old and new integrations do not replace each other, they can be connected at the same time and used both at once.
You can set up a new integration with Slack through the "Channels" → "Add a channel" → "Slack" section. Now the tile with the list of channels looks perfect - it seems that it was Slack that was missing in it :)