I noticed that after update (1.42) when I move then I cant look up and down fully just approximately 55 degrees. I must stop moving if I want to look up (and down) fully. I dont understand this restriction! As I do not understand why I can not look up and down 90 degrees! They are very annoying and not realistic! Where did this idea of the incomprehensible and who invented it?

• Failure to look down while being prone also causes failure to throw smoke just where you are lying, instead of doing so you are forced to throw it at your front. You just can't throw it at yourself, unless smoke has something to collide with at its trajectory. Sometimes it hasn't.
• The latter screen i did to somewhat demonstrate how you are unable to deploy at non-flat edges of terrain. Marksman showcase actually showcased lots of moments where you are unable to deploy your rifle neither on rocks, neither on grassy terrain, neither on acute-angled terrain. It also showcases frustration and ridiculousness, where "marksman" operates at a funny 100m distances still suffering from sway - which gets tamed by deploying, and who very strangely outperforms lots of guys with assault rifles which are made for distances like 100-400m.

I actually remember those tickets that requested to fix the ability to point guns into terrain, to run while looking at terrain, etcetera, that could have caused it. I don't think those tickets were caused by either pain or the great userbase struggle to bring moar realism, they rather likely were caused by useless pedantry. Despite the update i still think those tickets weren't fully "fixed", you can still poke your rifle into terrain textures, you just can't do it to a full extent, and you can still "break" some running animations by not looking forward.
The latter (ability to run and look into terrain at the same time and have "weird" animation) you could have even use in the past to gain some advantage while moving under obstacles without crouching. I did, sometimes. Because why not? Being inventive is one of the parts of winning, having advantages enemy hasn't - is too.

Infantry basically has no freedoms to move wherever it can. It also can't climb or jump without wounding itself. Another frustrating thing is general tendency there to balance things, not to give advantages which do exist irl between competing weapon systems, competing armies, vehicles, planes or at a smaller scale between competing privates (one private could just be stronger than another and thus suitable for wielding heavy stuff, another private could be a conscripted shy nerd who can't run long, aim well and bravely fight for his emperor.)

I think it is to avoid the very funny of animations while moving, often displayed with players are bored and waiting somewhere (e.g. on a chopper pick-up).

After all, running (or even moving normally) while looking straight up seems very awkward and not realistic (well, try running across an acre while looking straight up; you'll soon fall over straight).