Delete a file uploaded via field? Is it possible to delete a file not in usage outside of the context of a field widget in Drupal 8?

I tried to find a route like admin/file/{file}/delete but don't really see anything in the file module to do this operation.

I found this issue, which I assume is related. So the question remains: How can you delete files? I've set "delete orphaned files after" to Never in the admin so that files are not mistakenly deleted, which has happened to me a few months ago.
There is no functionality for this in core in the UI. You could do a entity load + delete with drush ev/core-cli or console, but that's basically your only option.

File entity does offer a UI for this, but you possibly don't want to use it anymore.

That said, setting that setting to Never is IMHO the wrong solution, as it also doesn't delete files that were only uploaded but then for example removed again or never really submitted. See https://www.drupal.org/node/2801777 for more information and another solution for that.

That said, when using media_entity, it should be save to configure Drupal to delete unused files as the media entity itself will always count as a usage of the file, so it will only ever get deleted if you manually delete a media entity.