The following refers to Classic/SD graphic only.
Huntress, Glaive Thrower, Shadow Hunter, Rexxar, Archer, Wind Rider, Guard Tower, Watch Tower, etc, and Fan of Knives’ projectiles used to have visible trajectories (not particularly distinct, but can be seen effortlessly), however from a certain version the trajectories became barely visible.
In this case, it could be assumed that most physical (keen?) projectiles without attachments (like “XX Arrows” enabled) with trajectory effects should have this problem.
Having a distinct trajectory can be more helpful in judging the direction of the attacks in chaotic battles, therefore the change could affects the gameplay somewhat in certain situations.
Similar condition occurred to Spell Breaker: not trajectory but the projectile itself. The fading extent is not as significant as the trajectories but the visibility is still reduced visually compared to very earlier versions.
Summarize: between opaque and translucent in very early versions, between translucent and transparent thereafter.
This is obviously harder to verify than the above since if someone is interested in verification, you’d be better looking for videos/versions that have been released for at least ten years, the earlier the more likely to be favorable for verification.
Here are some example videos for verifying. Perhaps it’s not the most proper but can be used.
Just pick some relevant Missions to check, or choose 41st and start watching from 2:07. These can also be used to verify the trajectories problem.
I wouldn’t rule out the possibility that these are some kind of tweaks, but personally, I’m tending towards them being some kind of bugs…