This morning, Apple has reinstated Big Sur upgrades on its software update service. However, users may still encounter errors if you try using the Software Update pane to obtain it.
There are two issues which complicate this. First, Apple has also pushed updates to XProtect and MRT, to Safari bring it to 14.0.1, and Pro Video formats 2.2.0; given yesterday’s problems downloading Big Sur, those appear exceedingly ill-timed. When your servers are in a deep hole, perhaps it’s time to stop making your hole deeper by releasing even more updates?
The other issue is easily worked around. If looking for the upgrade in Software Update reports incorrectly that an internet connection can’t be established, close that pane and locate the Big Sur upgrade in the App Store app. Oddly, this isn’t yet on its front page, but has to be searched for. Click on the Get button, and that should open Software Update, which can now find the upgrade and offer it to you. Download speeds now seem more normal than they were yesterday.
I wish you success.