“Backdoor-list.txt” files unexpectedly in server

If somebody is actively looking into your site to hunt malware, let them do the repairs first before you delete anything.

Those numbers look like timestamps, so my guess is that those are results from malware-scans running at those dates. It’s a text-file, you can download them and take a look at the content. Maybe add an (anonymized) example content to your question. Also, add information about your hosting company, because that’s most likely a custom configuration they are running.

Lastly: when in doubt, contact your host.