/scoreboard objectives add flight stat.flyOneCm
/scoreboard objectives setdisplay sidebar flight

when now glidinig with the elytra the number doesnt change, even when going upwards.

the wiki says that stat.flyOneCm tracks:
Distance traveled upwards and forwards at the same time, while more than one block above the ground.

either the wiki's definition of stat.flyOneCm is wrong or stat.flyOneCm.
i wager the wiki, as flying completely horizontal in creative increases the number.

stat.aviateOneCm tracks overall distance travelled with the elytra, not only horizontal distance (ie also falling after hitting space)

so, either include travel by elytra (only vertical) in stat.flyOneCm or change stat.flyOneCm to include all flying, not only vertical. or change stat.aviateOneCm to only track vertical movement like stat.flyOneCm. or create more stats based on flight. idk, as it is know its weird.