Too many bright-coloured UI elements on a screen ruin immersion

you can change the colour of most if not all of those UI elements

Colour changing wouldn't solve the issue of distracting UI elements. I thought of turning off some those UI elements. However, turning them off would make an advantage for those players who use all this excessive UI stuff.

Everything you are pointing out on your screenshots seem essential to me, i would just tweak some of the colours and reduce the size of some of them. Also, you can't complain about revive request icons as a support player

You can change: color, size, brightness? and opacity. I made all friendly icons really small and dim, gave them a deeper blue and darker green but left the enemy icons a bit brighter but overall smaller and gave them a more red color instead of the neon orange, flags and such i set to almost invisible. It made quite a difference in emersion, less people being hidden behind icons and less enemies being mistaken as friendlies. idk about if you can give the kill feed and such the same treatment(i know you can turn it off) but you can select a bit more condensed version of your squad info. Hit makers you can make very faint and you can turn off or move the damage numbers around to a spot just outside your focus so they are less distracting.

I believe the revive request icons should be visible on a mini-map and full-sized map only, not on the screen. This game isn't about some mech-warriors. Human soldiers dont't have a screen infront of their face. This UI looks just excessive and kill immersion. 

I always turn off the kill feed. I have no interest in that. I also adjust sizing and opacity on everything else I can. I prefer a lean, clean interface. 

You just missed my point. Those UI elements not just kill immersion, but give huge advantage to those who use them. Just look how easily this dude kills the enemies without even seeing them. This feature makes absolutely no sense.