In fact, there is EPC code, but it is rarely used and bank support is abysmal, at least in our country. But that can also be because we have some homegrown local standard for payment QR codes (and a new one in the works, lol).
I agree with you that the push towards them is annoying. (Google's "Your phone has new exciting features.")
In this case, Calibre does not seem to introduce any said annoyances (probably because it is FOSS, so no pressure for adoption), but people are upset anyways.
There are many features I don't use in various software, but it never made me complain that a new icon/menu entry appeared.
I struggle to understand the pushback against AI features. As long as the feature isn't intrusive, it seems like a minor addition, and may even be useful to some people. LLMs are here to stay, there is no denying that at this point.
Delta.Chat is really underappreciated, open-source and distributed. I recommend you at least look into it.
Signal, on the other hand, is a closed "opensource" ecosystem (you cannot run your own server or client), requires a phone number (still -_-) and the opensource part of it does not have great track record (I remember some periods where the server for example was not updated in the public repo).
But yeah, if you want the more popular option, Signal is the one.
[0] https://en.wikipedia.org/wiki/EPC_QR_code