I'm hearing from a number of people that some old versions of Acorn aren't working correctly on Big Sur. This isn't unexpected, as apps that haven't had updates in 5-10 years probably aren't going to work on the latest and greatest versions of MacOS. Every year Apple changes fundamental things in the OS, and it's a ton of work for developers to keep an app looking correct and running with the latest MacOS versions.
At the same time, I've been amazed that people have been using Acorn 3.x for over 8 years. That's … kind of amazing? Acorn has gone through all kinds of changes since then and I think it's in a much better spot these days. But I can't help but be wowed that it was still working great for people for so long. They probably know that version of the app better than I do at this point.
I look back at Acorn 3's single palette and I get a little nostalgic. It seemed such a simpler time (it probably wasn't, but everything is rosy when you look back).
If you have an older OS and want to grab and older version of Acorn, we keep all the previous copies available for download on our legacy page. If you bought it from us, you'll always be able to get it from us.
As of now, the only officially supported version of Acorn for Big Sur is version 6.6.3. But if you're on a previous version of Acorn and it runs great on your OS and you love it, you do you and keep on keeping on.