I’m downvoted but I still believe the position doesn’t make sense. You state and negotiate the level of protection you want for your data with your direct interlocutor. They then enforce that with their own suppliers.

You are free to want to know the whole chain of companies involved in providing you a service but that’s very unusual.

It's hardly that unusual.

https://cloud.google.com/terms/subprocessors

https://aws.amazon.com/compliance/sub-processors/

https://www.cloudflare.com/gdpr/subprocessors/

https://www.atlassian.com/legal/sub-processors

https://slack.com/slack-subprocessors

https://www.linkedin.com/legal/l/customer-subprocessors

https://grafana.com/legal/list-of-subprocessors/

Looking at your answer, I surmised it was a legal obligation and yes, it's indeed part of the European GDPR. I learnt something new today.