A Send a test message button beside the Slack webhook field on the Logging tab.
It posts to whatever is in the field — saved or not — waits for Slack's answer and reports it. Ordinary log traffic is fire-and-forget, so a webhook Slack rejects otherwise fails silently.
Requires lauzis/wp-plugin-packages ^1.16; this repo does not track composer.lock, so run composer update on deploy.