1. The fact that some designers ignore other devices than their own is definitely true. That is why I created a Figma plugin to broaden the perspective. https://www.figma.com/community/plugin/732240841094697441/Vi... But developers do the very same thing and as often as designers. Just further down the pipeline.
2. Not supporting the smallest possible devices like the original SE is a reasonable business decision. It is not even making it into the charts at all the products I'm currently working on. It is simply not worth the effort (since the effort is not as insignificant as people here say).
3. Supporting the smallest devices is definitely not free. And it has a real monetary, technological and UX cost. It costs a lot of money to properly test, report, design, and implement every possible edge case for every possible viewports. Developers HATE creating special layouts or cases for specific devices (speaking about native mobile development) and if I recall correctly, Apple might even prohibit it. And lastly, design is about making the best possible solution, for the biggest amount of people. Quite often you will find yourself in a situation where you have to decide whether you will vastly improve the experience for the 99% of people and make the experience not ideal for the 1% or vice versa. Which is the better call?
4. Blaming it all on designers is just too harsh. Yes, not everyone is perfect, and designers often miss it, but more often than not it is a simple business decisions. Designers can decide to ditch a certain viewport because it can benefit the majority, the same way developers decide to drop certain OS support.