Microsoft no longer supports any macOS before macOS 11.x Big Sur and soon macOS 12.x Monterey will no longer be supported.
In fact even if you had an older copy of MS Office, you can no longer activate those older versions because Microsoft has turned off those activation servers for older products.
If you previously "purchased" or downloaded the Apple apps like Pages, Numbers, Keynote from the App Store, then you can use them to work with older versions of any MS documents. If you never previously "purchased" or downloaded them, then you will no longer be able to do so now.
Otherwise you can use the free & open source LibreOffice. Even this option will require you to install and older version, so at some point there may be issues accessing documents created using newer versions of MS Office. The last supported version of LibreOffice for macOS 10.11 is v7.2.7.2 available here (page has links to various language packs as well if needed):
https://downloadarchive.documentfoundation.org/libreoffice/old/7.2.7.2/mac/x86_64/
Otherwise, here is the direct download link to v7.2.7.2:
https://downloadarchive.documentfoundation.org/libreoffice/old/7.2.7.2/mac/x86_64/LibreOffice_7.2.7.2_MacOS_x86-64.dmg
For anyone wanting to know what the most recent version of LibreOffice is compatible with various operating systems, here is a link to a chart showing compatibility for all platforms:
https://wiki.documentfoundation.org/Documentation/System_Requirements#LibreOffice_on_other_OSes