Installing Microsoft Office in older macOS

Yes I have been through it before, having bought a "lifetime" license for Microsoft Office in the early 2000s — specifically Microsoft Office v.X.

A few years later I learned what a lifetime license from Microsoft was worth. Since that time I abandoned Microsoft products altogether, and I recommend you do the same. The only difference between then and now is that there are many better options today. Some of them are free.

To my knowledge you can keep using that High Sierra version of Office for as long as you want... but it might not be able to open or otherwise manipulate files created by newer versions of Office.

If you ever need to reinstall the system & restore from a backup, that restored copy of MS Office may no longer work if it does not retain its activated license. From what I understand, Microsoft took the activations servers offline for older unsupported products.

LibreOffice may provide you with a more current option, but even LibreOffice is now outdated & unsupported on High Sierra. However, it will still install just fine if you ever need to reinstall or restore the system.

Here is a link to the download page for LibreOffice v7.4.2 which is the last version with support for High Sierra:

https://downloadarchive.documentfoundation.org/libreoffice/old/7.4.7.2/mac/x86_64/

Just for a general reference, here is an LibreOffice article for identifying the versions of the app compatible with various older outdated & unsupported operating systems:

https://wiki.documentfoundation.org/Documentation/System_Requirements#LibreOffice_on_other_OSes