A Send a test message button beside the Slack webhook field on the Logging settings.
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.
The endpoint carries a nonce and requires manage_options. Bundled lauzis/wp-plugin-packages updated to 1.16.0. 204 tests green.