No idea?

but to break the points apart a bit...

FM access would cut down on mobile data usage

FM radio is something like 54kHz. Simplisticly we can say that would be 54kbps... let's even say it's double that to account for overhead. so 108kbps. That's just about 1 GB/day. That's not really an amount that carriers sell anymore... Nor do people normally listen to music 24hours a day.

up-to-the-minute weather reports and automated alerts

Only if you're listening... and our phones do automated alerts already... as long as those systems are working. And considering that FM is a completely different well documented thing... it's worth keeping it around.

are broadcasted at 162.400, which means you need specialized radios to pick up the stations…

Nah, FM has been built into chipsets for well over a decade at this point. Most radios even still have them today. Just disconnected. But even if it wasn't available... Most phones at this point use RTL-SDR radios... software define radios means you can just program it to FM frequencies and it will work.

maybe an external antenna could use the shield or one of the pins on the phone’s charging port, or wired USB-C headphones and 3.5mm adapters could be updated to work as antennas…

Or even the wireless contacts for Qi charging and NFC... Just disable the radio app when these things are actually in use...

There could also just be an adapter that connects to the phone and contains all the required hardware…

No need, the vast majority of phones already have what you'd need. Just never connected/used by software for some reason.