Flags are (mostly) not part of the standard, regional indicator letters are. When you pick the German flag emoji on your keyboard, what you actually type is two characters: [Regional indicator D], [Regional indicator E]. The device what you typed is viewed on, usually chooses to display a German flag when it encounters the DE indicators.

So adding or removing flags is not up to the Unicode standard, but up to device manufacturers or library authors choosing to interpret the indicator letters in some way. There is nothing stopping Samsung displaying a vegan flag when the indicator letters VEG are used — though you can also argue “vegan” is not a region.

Some devices can also choose to display a joined sequence of other emoji as a different emoji. Rainbow flag emoji is not in unicode, but many devices display it when the White flag and Rainbow emojis are joined together. So that’s another way to achieve it, maybe White flag + Green pepper could be joined to a vegan flag. But they would also not be up to Unicode, but up to devices choosing to display it that way.