macOS Big Sur and bash
In Catalina, Apple made ZSH the default shell, however, macOS retained the (now deprecated) Bash.

There's some misconceptions that need to be cleared up:

ZSH is the default for shell for new users in Catalina, however, for existing users, the Bash shell remains the users' current shell.

Bash is not deprecated in any way. It is still current and the version that ships with macOS is perfectly fine.  The reason that Bash is &quot;so old&quot; is because of the GPL license version.  Apple switched to ZSH for this same reason.



Is there any official word out there regarding this? At the very least (...and FWIW), any anecdotal experience coming from beta installations?

There's no &quot;official word&quot; on something that's not even projected to happen.  As for anecdotal evidence, I cite cron;  it has been deprecated since Snow Leopard in favor of launchd. We're in Catalina now an cron is still around and fully functional.
It's also important to note that there is a difference between deprecated and no longer supported.

cron is deprecated meaning that while still supported, there's no more development on it and admins/users should start migrating to the new tool(s) (launchd).
AFP is no longer supported for file sharing.  You can use it as a client (connect to an old server with AFP support), but you cannot share a folder from Mojave/Catalina with AFP.

Just because Apple makes a move to support a more modern technology, doesn't mean they are going resect it from their base code so it's not longer available in future versions.