You're looking for the Office 2021 proofing tools offline installer. Here's what I found:
The proofing tools for Office 2021 include language-specific dictionaries, thesauruses, and grammar checkers. These tools can be installed separately from the main Office installation. office 2021 proofing tools offline installer
Unfortunately, I couldn't find a direct download link for the Office 2021 proofing tools offline installer. Microsoft often changes its download links, and I don't want to provide an outdated or incorrect link.
Please note that the above method requires some technical expertise and might not be suitable for all users. You're looking for the Office 2021 proofing tools
If you're comfortable with command-line installation, you can use the following command to install the proofing tools: