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 Microsoft often changes its download links, and I don't want to provide an outdated or incorrect link.
However, I can suggest an alternative:
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
If you're comfortable with command-line installation, you can use the following command to install the proofing tools: Unfortunately, I couldn't find a direct download link
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: office 2021 proofing tools offline installer