Sub menus are really hard to do right on a mobile device.

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.