How do I access old word 2011 for mac .docx documents if I upgrade to Mojave? Is read only access still supported?

Unless the Word document is damaged, Pages '08, '09, and versions 5 thru 7.2 will open .doc and .docx documents. However, no version of Pages is a Word clone, and though it will open Word documents, they are translated into Pages internal document format without changing the original Word document. The translation is done in reverse when exporting to Word .doc and .docx formats. You will not be able to edit, or save the original Word document.

Features unique to Word and Pages will interfere in document translation.

I didn't see anywhere that you wanted to open with Pages, but it doesn't open .doc files (old format).

There is no way to tell if Apple will drop that support.

If I wanted to keep such files, I'd convert them to a current format, like PDF.

Using Office 2016, I can open Word documents created prior to them switching to the Open Document format.