What is the last Mac OS that will run Micosoft Office 1011?

Microsoft never tested Office 2011 for Mac with High Sierra as they retired it in Sept 2017. However, users indicated that it mostly works in High Sierra or Mojave, depending on what features you use. Mojave is the last version of macOS that supported 32-bit applications and that is what Office 2011 provides.

If you do not have Microsoft Office 2011 for Mac currently installed, then you won't be able to install it on the older operating systems as 1) Microsoft blocks the installation of retired products and has disabled the relevant license servers, and 2) Microsoft only supports Big Sur, Monterey, and Ventura for current installations of Microsoft 365, or Office 2021 for Mac.

See BDAqua's comments, or consider a better Microsoft clone interface and native document compatibility with the fee-based Softmaker Office 2021/2024 for Mac product. It has a free trial and a choice of subscription or single-purchase. Supported on macOS High Sierra (Office 2021) or macOS Mojave (Office 2024 - beta).

Microsoft shut down the activation server for Office 2011 at least a couple of years ago. If your plan was to install it fresh after downgrading the OS, you won't be able to activate Office.

Office looks in your user account for files it creates to tell it whether or not the suite has already been activated. So, you do have a workaround if you can do the following. There's more than one way to do a backup, but I'm going to use Time Machine in the instructions.

1. You absolutely must do this first. Create a restorable backup with Time Machine while you're still running Mojave. There is no way to go back to an older OS without erasing the drive, which is why you need the TM backup, first.
2. High Sierra can sort of run Office 2011, but it's hit or miss. It would be better to go back to an OS no newer than Sierra. Erase the drive and install the OS.
3. When it gets near the end, the installer will ask if you have data to restore. Choose yes and point it to your TM drive.

I have no idea, I'm not even sure they let you runOffice 2011 anymore.

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

https://www.libreoffice.org/

Thanks to Matti Haveri…

MS Office for Mac Mini - Apple Community

LibreOffice 4.0.6.2_MacOS_PPC works in OS X 10.5.8 (last official PowerPC version).

LibreOffice 4.3.7.2_MacOS_x86.dmg (and x86-64?) works in OS X 10.6.

LibreOffice 7.0.4 works in macOS 10.11 and 10.14.