My company uses a self-hosted Slack clone called Mattermost. It is generally compatible with Slack webhooks.
The Mattermost webhook URL has the format:
https://subdomain.domain.com/hooks/xxxxxxxxxxxxxxxxxxxxxxxxxx
Attempting to set up a Slack notification with this webhook URL fails with the message “Disabled Automatically by Buildkite: Invalid Slack URL”.
Please consider allowing Slack style notifications to Slack compatible (but non-Slack) webhook URLs. Thank you.