The arrow aren't suddenly slowing down. They also aren't deciding mid air to change the arc and speed, thats gravity already doing for them. They slow down the moment they get shot by gravity and air resistance. On the highest point they are the slowest and tip over. As soon as they start to fall they gain momentum and then are stable again. The visuals maybe don't line up with reality 100% since they should speed up a bit while falling down, but the game engine doesn't really do this with other projectiles either. In general arrows fly in a parabolic trajectory like seen in this gif.

Covering parts of the screen won't change anything about the parabolic trajectory, thats just basic physics and maths. And the game probably just uses this math to create the trajectories of projectiles. The second gif is alright, although hitting something with such slow arrows is like hitting someone with a wet noodle.

As a test you can cover 75% of the right side of gif and predict where those arrows would land that you see starting on the left.
You can do the same on the other side and cover 75% of the left and predict where those arrows came from that you see landing on the right.

(that is all besides of course that all this wouldn't deal any damage and wouldn't be something you see; they would rather try to shoot closely over the heads of their allies at this distance, if they dared)

You do not see the arrow suddenly slowing down at more at ~25% or it's travel distance in order to continue in a homing (slow) motion? It is like they decide mid air that they actually wanted to change the arc and speed. It is not on the same arc or speed it had initially.
It is more obvious in the second gif I shared below. Those are not arcs the arrow would take. At this distance, they would either overshoot or fall down/flip over instead of homing in on the target. The arrow needs some speed to do this kind of arc.

Yeah I watched this a few times now, but I don't understand what you mean.
During the entire flight the arrow fights against two things. Air resistance and gravity. An arrow is usually effected by wind too, especially if they are slow like this, but its not modeled ingame, so the flight trajectory is even more predictable. What you're seeing here is essentially just a parabolic arc. At the top its the slowest, and because arrows are front heavy they tip over like that (or tumble) if they get to slow.
Also why should the arrow get far further than this? An arrow shot half-drawn (or less) is really slow, so arrows won't fly further than a few dozen meters.

Here is an even stranger example )) https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3184179956

If you follow the trajectory of a single arrow in the picture you will see how it suddenly -mid air- hits a resistence that wasn't there before, just so that it can cut short the otherwise expected arc it would have went, far further than those enemy units.

Of course arrows fly like that its a parabolic trajectory. If you shoot them like that they are the slowest on the highest point of the trajectory and then they speed up due to gravity, they still have their forward momentum and the feathers at the end of the arrows keep them stable during flight. Its physics.

If you wanna read about half-drawn arrows, check this out: https://bowvsmusket.com/2015/02/27/baron-marbots-encounter-with-mounted-archers-at-dresden-and-liepzig-1813/
Stuff like that apparently happend occasionally, though in his memoirs Baron de Marbot describes that its wildly ineffective. The French essentially only got minor injuries despite being totally unarmored.

I am happy to be corrected but I doubt that anybody would shoot an arrow like this at such close range. I also do not think the arrow would fly like that. In the example it is homing in on the units below but in reality with this initial arc and speed it would go much further and not have change direction and speed -in such way- mid air.