I ride in the city without a flag.

I can't prove it, but maybe the "too low to the ground" debate is inspired by what regular upright cyclists feel about their own visibility. I don't think that the visibility of any bike on the road is a problem at all (in daylight at least) - the real trouble is that the general profile or appearance of the upright cyclist is so common that it is taken for granted and subconsciously ignored.

The distinct look of a recumbent is eye catching enough. I get much more space and respect on the road compared to my upright bike.

If you do a lot of filtering through moving vehicles and the top of your head/helmet is below the level of vehicle windows and so close that the driver cannot see you, then maybe a flag is useful. But perhaps this sort of cycling strategy is to be discouraged anyway.

Or buy a flag if you like flags in general. For sure they are fun to look at!