Chrome/Chrome OS Version: Whatever was running on the AOpen Chromebox Mini on December 28th, 2017.
Chrome OS Platform: AOpen Chromebox Mini
Network info: not sure, probably doesn't matter.

Please specify Cr-* of the system to which this bug/feature applies (add
the label below). ← I don't see a button to do that anywhere.

Steps To Reproduce:
(1) Go to drive.google.com, and create a file with a slash on it: "foo/bar", for example.
(2) Open the ChromeOS file explorer, find the file.
(3) The file displays as "foo_bar".
(4) Try to rename it as "foo/bar" from the file explorer.
(5) It silently fails without explanation.

Expected Result:

I can put slashes in file names.

Yes, I know, as CS person, this is a terrible idea to have slashes in names. However, especially since Google Docs file names are automatically generated from the document title, slashes are going to end up in there. So ChromeOS should handle them gracefully in the file explorer.

Actual Result:

Slashes don't appear, renaming silently fails.

How frequently does this problem reproduce? (Always, sometimes, hard to
reproduce?)

Always.

What is the impact to the user, and is there a workaround? If so, what is
it?

It's confusing. No workaround for the "slahes displayed as underscores" problems. Workaround to rename file to have slashes in them is to do that in the Google Drive Web UI in Chrome.