GrapheneOS stops supporting phones when Google stops releasing new firmware for them for security reasons, so you aren’t going to get longer term support from it.

They do have extra 2 years of partial "transitional" support, if I'm not mistaken. But yes, even they will not support them forever as it's not financially feasible without help in the form of first-party updates.

To do anything else would be snakeoil at best: no open source project can support the radio firmware

Is it technically possible to write an open source firmware for the existing wireless chips?

Technically, yes. The big hangup is that spec sheets for these chips are often hidden behind NDAs, so devs are left reverse engineering existing firmwares. This extra barrier to entry hobbles further development for many phones.

Not really, modem firmware even for wifi and BT is pretty much tied down "thanks" to regulations, you won't get far without an exploit bypassing the signature verification. Governments are apparently very afraid of people modifying their chipsets to be more powerful in range than allowed.

Could you link me some of those regulations or a article on it. I am curious

https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A... basically, they use the firmware to enforce compliance with the regs on different regions. If you can load your own firmware, you could make the device no longer compliant.

I have been nominated seven straight years for hacker news reader with the least knowledge of security, so I’m embarrassed to ask: what keys would one want to sign and why?

It’s not your keys, it’s the Operating System keys uploaded to your device.

Essentially the same way UEFI secure boot works in the PC world.

You’re telling the device hardware “it’s only ok to run software that’s been signed with the private key that matches this public key”, so that once you’ve done that, you can have confidence that the operating system hasn’t been modified in future by anyone other than the original vendor (as only they have the private key).

That's a perfect explanation. Thank you.

The keys that verify that new boot software is allowed, the ones that verify it's coming from _you_ and not some other asshole trying to take over your phone after you rooted it.

Ah, relief. Thank you.