There are two issues here (1) The stupidity of discontinuing an essential aspect of functionality of a Mail program, and (2) The poor customer service response of Apple.
It took a year during which multiple complaint and extensive customer feedback forced get Apple to reverse this change, but unfortunately they have imposed a complete no communication policy with their customers, which frankly comes across as hostile.
The result is that, like many of us, I am still on 10.5.2 and there are several apps that I can't update as they are not compatible. I am over-joyed mail columns are coming back in Big Sur, or so it is said (I look forward to getting my old Mail back), but although I scan update details, the web and various places, I have seen no confirmation of the fact, so I won't upgrade until I have proof that its there!
Apple - do us all a favour - without judgment or blame - just one sentence will do.