Slack integration stopped working

The recent change to the Slack integration on 7.4.0 broke it completely.

  - Changes to Slack service structure, only webhook url needed

Not only does the usability suffer as now you'll need to generate a new integration in Slack for each project / channel. Instead of just copypasting the one key to each project and typing in the channel. And change the icon and name for the Slack bot for each webhook.

But more importantly, it doesn't even work. I've created an incoming webhook in Slack:

And copy-pasted the unique webhook url to the project in GitLab, but neither the test button nor pushes to the project send any messages to Slack.