The /effect command cannot apply certain parameters like "ShowIcon" and "Ambient", even though they exist in ActiveEffects and CustomPotionEffects nbt

The bug

In CustomPotionEffects and ActiveEffects tags on mobs and potion items, you can apply effects with "ShowIcon" and "Ambient", which cause the icon to be shown or not, and show a blue outline around the effect, respectively.

However, unlike ShowParticles, these parameters cannot be specified to be used in the /effect command. 

Having these as options would allow mapmakers to do so much more, as at the moment you are restricted to area effect clouds for applying effects with blue outlines and hidden icons.