There’s already a number of existing feature suggestion threads discussing those options.
- Bring back 'Play Version' prompt dialogue for movies with multiple versions (4k & 1080p etc.)
- Players should already have a
Play Versionbutton – although it might be under the item’s context menu (⋮) - Not sure I get that point… you can set different target qualities for local vs. internet streaming on the Apple TV as well (at least it used to be this way last I checked… my Apple TV is currently broken)
- The multiple labelling does not exist on the cover anymore
There’s a feature suggestion asking to bring that back
- a popup to select a version does not exist anymore
It does exist but it’s no longer automatically popping up. IIRC users with many multi-version items had complained about that.
There’s a feature suggestion to bring the old behavior back (see #1)
- No matter how many versions a movie has, only the highest version is shown in the main window
From what I can see, Plex will display all versions on the movie page – however it defaults to the highest quality version. You have to click and X more to see the list of all available versions.
As you pointed out in your example… that can be a rather long list, so I understand why Plex wants to collapse the list by default 
- You have to select THREE POINTS each time to even see if there are multiple versions
There’s also the Versions label on the main page, next to the available audio/subtitle tracks.
No need to open the movie’s context menu (⋮)
- The information looks different everywhere. There is no unified view on all platformers.
I don’t have all those versions available. The ones I could check are usually showing the same amount of information (resolution and average bitrate of each version).
There’s a suggestion to show more information on the version selection popup.
