Pocketbook very recently added it. But most importantly koreader has it (an open source reader app that is available for a lot of devices), so I basically had it before (excluding system menus which was annoying).

Yes and no. For stuff like citations: absolutely. For reviewing stuff, the mode to suggest edits in Office (or even Google Docs) is great and doesn’t really have equivalent with a proper UI for LaTeX. Yes, you can use PDF comments, but then you need to change the LaTeX document manually.