a way to retrieve metadata of a previous version of a file? (using CSOM in PowerShell in SPO)

Brent Ellis have a look at this update https://dev.office.com/blogs/new-sharepoint-csom-version-released-for-Office-365-august-2017 and especially those classes:

public property Microsoft.SharePoint.Client.ListItem.Versions

public class Microsoft.SharePoint.Client.ListItemVersion

public class Microsoft.SharePoint.Client.ListItemVersionCollection

If they do not work for you, there is a way to do it from the _vti_bin/Lists.asmx service.

More info here: http://blog.velingeorgiev.pro/how-get-list-item-field-versions-data-sharepoint-online. It is not first class solution, but getting version metadata was not available in CSOM few months ago, I do know know if the august 2017 update will offer more on versions metadata (haven't tested it yet). Let me know if works, thanks!