office 11 on OS 10.13.6

FWIW:

• Office 2011 for Mac is a 32-bit application suite. Office 2016 and 2019 for Mac are both 64-bit suites. Office 2016 for Mac is no longer a strategic solution as Microsoft has stated that they plan to end support for it on Oct 13, 2020. Thus, replace Office 2011 for Mac with Office 2019 for Mac.
• Microsoft never tested Office 2011 for Mac on either High Sierra, or Mojave, and since that office suite was retired in Oct 2017, if anything fails to work on Mojave, you are without an update fix.
• Data migration between Office suites are typically supported, but that does not mean you may not run into some form of compatibility/feature issues upgrading.

Ref:

• Microsoft Product Lifecycle - Microsoft Support
• System requirements for Office - Microsoft Support
• Microsoft Office support for macOS 10.13 High Sierra - Microsoft Support

Office 2011 is no longer supported by Microsoft at all.

As Microsoft follows Apple's 3 year sliding window for OS support, Microsoft will not support any version of office running on High Sierra once Big Sur is released.

Your current documents will be the same as always. But you need a newer version of Office that will work. That would be either Office 365 (subscription), or Office 2019 (one-time cost perpetual license).

Since you're on 10.13 now, either of these is already on the last version of macOS Office 2019/365 will support. It won't stop working the day Big Sur is officially released, but it's unknown if Microsoft will make any more updates for 2019/365 to fix any issues that crop up in 10.13.

Look into the Free LibreOffice, it handles MS Office files like Word & Excel…

https://www.libreoffice.org/