MBA 2015 plugged in all the time

Yes, you can. No, it won't prolong the battery's life. Occasionally, you need to take it off the charger - about once every two months. Otherwise, the battery may die a little earlier.

About Batteries in Modern Apple Laptops

Battery University

Apple - Batteries - Notebooks

Apple - Batteries

Extending the Life of Your Laptop Battery

MacBook and MacBook Pro- Mac reduces processor speed when battery is removed while operating from an A-C adaptor

Apple Portables- Calibrating your computer's battery for best performance

Mac notebooks- Determining battery cycle count

It is a good practice to run it on the battery for at least one battery cycle per month without deeply discharging it. One cycle would be discharging it to roughly 50% twice in a month, or 4 times to roughly 75% or any other combination where it is used to roughly 100%.