I understand it's maybe a bit of an uphill battle to support a software stack that the OS/hardware vendor itself doesn't support, but:
1. telegram/whatsapp do, so it's definitely possible.
2. The point is not to add new features, but just to leave the current version be without deprecating it.
There is probably a bit of a development burden in keeping old versions up, but I don't think it's that big: they could even let the community take care of that…
3. But most importantly, "the iphone 6 had a good run", so the natural continuation to that sentence is "so let's discard all those working devices" ? wtf?
I think that from a company that defends "public interests", some more thought should be given to not wasting energy, time and money forcing people to upgrade working smartphones…
My guess would be that they want to dump the messy and easy-to-screw-up CommonCrypto code and switch fully to CryptoKit, which was introduced in ios 13.
If you don't like their way of running their project, you have other options.
I think you have an idealistic interpretation of Apple's PR. No one is forced to upgrade, your phone presumably still works as a phone and as a camera, music player, etc. Apple has nothing to do with Signal. They do offer a trade-in program, though an iPhone 6 won't get much.
What wouldn't make sense would be to spend resources to continue supporting an unsupported operating system with an imperceptible market share.
The iphone 6 had a good run (2014-2023), but it's time to upgrade.