The issue is not with Catalina Bud, it is with Big Sur. BIAB compatibility with Big Sur.
I totally understand all of the Big Sur issues. I just read the post as the issue being with Catalina. One could interpret the statement as referencing Mojave v Catalina.
“ Still talking about a dual boot on Macs to load Catalina to run BIAB...”
Old phart moment I guess. They come more and more frequently:)