You're looking for the Office 2021 proofing tools offline installer. Here's what I found:
If you're comfortable with command-line installation, you can use the following command to install the proofing tools: office 2021 proofing tools offline installer
However, I can suggest an alternative:
You can try using the Microsoft Office Deployment Tool (ODT) to download and install the proofing tools. ODT allows you to download and install Office products, including proofing tools, using a command-line interface. You're looking for the Office 2021 proofing tools
setup.exe /configure proofingtools.xml Assuming you've downloaded the ODT and extracted it to a folder, create a proofingtools.xml file with the following content: including proofing tools