I have found this: Why does Mac OS include both ABC and US keyboard layouts when the two perform the same functions?, where the answer stated:
ABC is just the same as US but you don't have an American flag. Some users outside the US may find that preferable.
However, this answer doesn't stand anymore, since the latest versions of macOS has used the A symbol for any English keyboard layouts if you only have one of them available. And they would only change to language/nation abbreviations if you have more than one English keyboard layouts(not flags anymore).
So my questions is, are there really no difference between ABC and US keyboard layouts now? Or are there something I'm not seeing?