Yes, I hadn’t even migrated an old shell. I was formerly on bash and decided to take clean, fresh plunge to use Apple’s Zsh. I’d installed a bunch of stuff and then Big Sur update killed (overwrote?) the symlinks and extra path variables and config that had previously been working fine in the .zshrc file.

I feel like that might have been a one off, and perhaps future updates won’t be the same. But at the same time I had to draw the line somewhere, so we’ll se if this appears anywhere else on the internet in people’s experiences.