My nonfiction manuscript is in MS Word 365 with 28 pages of endnotes at the end of the last chapter. How to separate endnotes into its own section, so can set "Notes" as the header for those pages. Currently they inherit the odd/even headers of the last chapter?

Each chapter is its own section with headers set to Different Odd & Even Pages: part title for even page headers, chapter title for odd page headers.

A related back matter header problem: I have successfully created new sections following the endnotes for the Bibliography and Index. I successfully set the the odd number pages headers in these sections. However, they inherit the last chapter's even number pages header, despite turning off the Link to Previous setting. All attempts to disable Different Odd & Even Pages in these two sections also disable the Different Odd & Even Pages headers setting in all chapters of the book. What am I missing that would fix this?

You could simply insert a Section break before your endnotes, then unlink the endnote section's headers/footers from the previous Section so the endnote section can have its own headers/footers.

For advice on inserting content after endnotes, see Inserting material after Endnotes, especially posts #12 & #18.

As for the 'Different Odd & Even' issue, simply accept the fact that that's what you have to work with and, if necessary, insert the same header/footer material for the odd & even pages.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thanks, macropod, for you helpful reply. Turns out that creating a separate Notes section solved by:

1. Create new empty section after last chapter
2. Suppress endnotes in all chapters, but not in the new section

Suppressing headers in sections not a well advertised technique from my Internet searches and Word Help searches. But I found this very helpful guide by the University of Wisconsin.
https://lafollette.wisc.edu/images/p...placement.docx
To suppress endnotes:
1. Click in a section.
2. Go to Page Layout: Margins.
3. Pull down to Custom margins.
4. Click on Layout tab.
5. Check suppress endnotes.
6. Repeat for every section except for the last one that starts with the continuous section break after the endnotes header.

Solved the back matter headers problem by: unlinking each section's first odd and even pages from previous section, keeping the Different Odd & Even pages setting, setting each section's odd and even pages to the same header.

You should not need to unlink every header and footer, just the last section's ones.
Look into using the StyleRef Field.