Windows - Portable (.zip): https://mirror.clarkson.edu/blender/release/Blender2.81/blender-2.81a-windows64.zip
I got these links using the tool Wayback Machine (https://web.archive.org/). It has a saved Blender download page from few days ago (https://web.archive.org/web/20200125185852/https://www.blender.org/download/). When you click on download it appears to you a "thank you" page, then there is a link on a text on the bottom of that page ("Your download should begin automatically. If it doesn't, click here to retry."), which leads to an official mirror download. You click on it and the download will start.
In case www.blender.org is completely offline, you can also download it from the following official sources:
- Steam (requires local installation of Steam)
- Windows Store
- Ubuntu package
- One of the official mirrors
Alternatively, if you're a software developer, you can clone the Blender repositories and build it yourself. This will allow you to compile Blender from past commits and releases locally, if you have the required dependencies installed, even when all Blender websites are offline. In case you're planning to prepare for potential outages, you may want to clone the repositories for the manual and API documentation as well. Blender also has a mirror of the source code on GitHub.
When the official website is reachable, but the download is slow or fails, you can use the click here to retry link at the bottom of the "Thank you"-page. This will use one of the mirrors that is geographically close to you, thereby reducing round-trip times.