Since 3 July 2020, https://github.com/os/slacker/ says:
This repository is archived and will not receive any updates
It's time to say goodbye. I'm archiving Slacker. It's been getting harder to find time to maintain this project for a while now. For years it's been the most popular Python library for Slack. Eventually Slack decided to go with their library, and I lost my motivation to maintain it. Thank you all for your contributions to this project.
Whilst slacker-cli + slacker is still working, slacker-cli should consider migrating over to https://github.com/slackapi/python-slackclient.
Since 3 July 2020, https://github.com/os/slacker/ says:
Whilst slacker-cli + slacker is still working, slacker-cli should consider migrating over to https://github.com/slackapi/python-slackclient.