Where I can see all modifiers that are gained for getting deeds or appropriate technology that give Empire Improvements? I've searched under Empire tab but they are nowhere to be found.

I don't think there is such info in-game, but there is a complete list on the community wiki.

I think it's weird that we can see many modifiers, but this one is hidden - since the very beginning.

Yeah, I was wondering this as well!

Endless Legend tacks this stuff onto the tech tree. I'd like to see what I've got in ES2.

Is there a rational cause that it's still not implemented? 

Somehow this topic slipped my radar all this time and I've never really gone looking for such things.  Now that you mention it, it WOULD be nice to see acquired traits, including any empire wide techs as well; it would assist in better analyzing our games.

Still, it's been 7 months and nothing happened. 

Part of Megathread:  https://www.games2gether.com/amplitude-studios/endless-space-2/forums/66-game-design/threads/34614-lore-of-awakening-damage-control-mild-to-radical-proposals-a-cheeky-attempt-at-megathread?page=1 

+BUMP.

It would be nice to be able to see all those `empire improvements` that are given via *quests* and other events...

Only way I've found is to look at Completed Quests for the ones that are quest rewards.   Really bizarre this isn't something readily available from the Empire tab.