Bootstrap. The people who removed sub-menus from Bootstrap 4 because it was deemed "too difficult". Not cool.
Mobile is important at least since Bootstrap 3, when Bootstrap became "mobile first" (e.g. you define breakpoints with the smallest screen in mind by default, adding exceptions for wider screens)
After all, Bootstrap is, like the name suggests, a "bootstrap" for your own project, and if you think the accesibility problems that come with nested dropdowns are worth it, you're totally free to implement that, and even create a GitHub project with your CSS clases.
There's no reason your solution wouldn't be merged if you provide a truly novel approach to all the other failed approaches.
No, they are not. Bootstrap is too opinionated here by not supporting nested menus. There are plenty of use cases for them, a navigable sitemap or your common application toolbar for example.
And accessibility is also a non-issue, see W3C's / WCAG's recommendations: https://www.w3.org/TR/wai-aria-practices/#menu
Speaking of accessibility: I'm surprised that Bootstrap only cares little about this important topic, especially considering implementing it on websites is required by law in many countries.
Nested menus are no easier to make generic. Or they can easily be too opinionated for the Bootstrap team's tastes.
Also I don't think dropping something from your API makes you more opinionated. Their solution would be opinionated by definition if they had one. Nothing stops you from building your own or anybody from building a library that solves the problem. If you think it's easy, then you sound like the right person to take on the challenge.
This is very opinionated.
Agree 10x