Windows Migration Assistant for macOS Mojave or later link is broken (connection not secure)

You were not alone...

Migration Assistant for Windws -- Users report "certificate Revoked"

Several users in MacBook Pro forums are trying to Migrate from Windows using Windows Migration Assistant. They report the software is unusable because it reports "Certificate Revoked".

All versions from this article download fine on my Mac.

Move your data from a Windows PC to your Mac - Apple Support

I am not a Windows user, so I can not help any further:

Trying to download the migration assistan… - Apple Community

Download migration assistant - Apple Community

I just tried this in the Brave Browser...

https://updates.cdn-apple.com/2020/windows/061-71710-20200518-65B46474-996E-11EA-AB94-605C1553BE55/WindowsMigrationAssistantSetup.exe

It worked. https://brave.com/

Migration Assistant should be installed in your Applications folder since it is part of your operating system. Have you tried using that? It should work just fine with your OS.