Yes, tcsh was the default in 10.0 then in 10.3 they changed it to bash.

I still not-so-fondly remember when csh was the default shell, on a real BSD-from-Berkeley can't-call-it-UNIX. :-P

It's still in there. The linked article tells how to do it.

In Terminal, enter $ chsh -s path, where path is one of the shell paths listed in /etc/shells, such as /bin/zsh, /bin/bash, /bin/csh, /bin/dash, /bin/ksh, /bin/sh, or /bin/tcsh.

And you can use MacPorts or (presumably) Homebrew to install newer versions of tcsh or pretty much any other shell you might want.

I personally never stopped using tcsh after the bash switchover since it's what I knew and what my dotfiles were already written in, and I'll most likely just keep using it in Catalina too.

Indeed. The point was it was the default shell, being true to its heritage.