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.
However, I can suggest an alternative:
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.
If you're comfortable with command-line installation, you can use the following command to install the proofing tools:
Please note that the above method requires some technical expertise and might not be suitable for all users.