I had to go and read the docs to confirm this was true, I'm really surprised this has been removed (since Raspian Bookworm apparently). It was a ridiculously common way to configure wifi on RPis over the last decade!

It's the little paper cuts like this that really hurt on a platform who's original aims were to target the education market, there is still a mountain of tutorials online advising to drop in that file for wifi setup.

I recently needed to create some heavily customized pi images for a fleet of Iot devices. I came across this tool that did the job nicely. Only downside is that it only works on Linux.

https://github.com/gitbls/sdm

Good news:

rpi-imager --cli

Has really expanded options over the previous versions.

Including:

--first-run-script <first-run-script> --cloudinit-userdata <cloudinit-userdata> --cloudinit-networkconfig <cloudinit-networkconfig>

Though it doesn't support the old simple config text file, presumably most of that is replaced by cloudinit network config.