A couple things here:
app.redash.io runs a modified version of V9 (not V7).
When you migrate from app.redash.io to OSS, the secret fields are not transfered. That includes the webhook URL for slack alerts. This is lightly addressed in the
redash-migrate documentation for moving data sources. But the same applies for alert destinations.
This step is necessary because redash-migrate uses the Redash REST API which never sends secret data source fields in plain text. It’s a security measure to prevent exfiltration of your organisation’s database credentials should a hostile party obtain an admin API key.
You need to get the webhook URL from Slack and enter it into your OSS instance.