Retaining .doc files rather than .docx files in a SPO Library

Kathy Grube 

DOCX

DOCX is part of Microsoft Office Open XML specification (also known as OOXML or OpenXML) and was introduced with Office 2007. DOCX is a zipped, XML-based file format. Microsoft Word 2007 and later use DOCX as the default file format when creating a new document. Support for loading and saving legacy DOC files is also included.

DOC

DOC is the default format used with Office 97-2003. DOC is a Microsoft proprietary Binary Interchange File Format. Microsoft Word 97-2003 uses DOC as the default file format when creating a new document. Support for loading and saving DOCX files can be added using an add-in.

https://support.office.com/en-us/article/file-types-supported-for-previewing-files-in-onedrive-sharepoint-and-teams-e054cd0f-8ef2-4ccb-937e-26e37419c5e4

https://www.techmikael.com/2014/01/keeping-versions-when-upgrading.html

I did a practical analysis on this issue, where I copied a doc file to SharePoint library & it still supports to download 
Copied below file to SPO Socument Library

Correct me if I'm wrong in understanding