What regulations would require prior notice of a sale?
I don't know. I think I did not phrase my response very well. Here is what the article I read said.
Since the pandemic started, Buffett has taken different approaches to the airlines. In March, he acquired more of Delta’s shares. However, as the outlook turned grimmer, Berkshire started selling his stakes in Delta and Southwest. At the beginning of April, Berkshire sold 2.3 million Southwest shares and 13 million shares of Delta. It turned out that these sales were aimed to reduce the Berkshire stake below 10%. This allowed the company to continue selling without the need to disclose the news in SEC filings.