Where did admin CSS file "ie.css" go after Django 1.8?

I'm in the process of upgrading a large Django project from 1.8 to 2.2.  Currently, I'm at 1.9.13.  My project has a couple of admin forms that utilize a Django CSS file admin/css/ie.css.  In Django 1.8, that file was located here:

django/contrib/admin/static/admin/css/ie.css

But in Django 1.9.13 that file is missing.  In it's place is a 'forms.css' file.  Does anyone know what happened to the old ie.css file?  I've searched online and also all 14 of the release notes for Django 1.9 and I don't see this mentioned anywhere.

Thanks.

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