I added Sentry's cron monitoring into my code, and the check-ins created a cron monitor in Sentry's Web UI. I have now removed the code, but the monitor remains in the web UI, and is reporting monitor failures.

Cron monitors are not automatically deleted from your Sentry organisation after you remove the code responsible for sending check-ins to Sentry.

To stop seeing the failures, you need to either delete or disable the cron monitor in the Sentry UI in Monitors -> Crons.