Even for OSX its kinda iffy as statements go. I think (been a while now) tcsh was the default for < 10.3, 10.3 was zsh and maybe 10.4, then it moved to /bin/bash as default shell.
Also, I didn't know that zsh was ever the default? I thought it switched from tcsh to bash around 10.3 or 10.4.
10.2 was tcsh, 10.3 was tcsh for user shells and /bin/sh for scripts.
I think 10.4 was the zsh experiment, then 10.5+ bash. We'll see how long bash lasts being a gpl shell.
Just giving historical context, not trying to say anything about markeshare or anything. Apple seems really willing to change shells is basically all my post amounts to.