I have retested this on my Big Sur installation. Under the System Preferences > General panel, setting “Prefer tabs” to “Never” returned to the behavior under macOS Catalina, and the vmods I checked functioned properly.
I have retested this on my Big Sur installation. Under the System Preferences > General panel, setting “Prefer tabs” to “Never” returned to the behavior under macOS Catalina, and the vmods I checked functioned properly.