There was "Slide & Change The Row" thing in the list editor. Please can you bring that to new list editor?

But when I add a new movie to my list to the 50th place then I need to change 51, 52, 53,... 100th movie. When I add that movie there will be 2 movies in 50th place.

Do you know anything about this feature? Can it be come back?

You said "When I add that movie there will be 2 movies in 50th place."
I have never seen that happen. I do plenty of ordering of my lists, and when I assign position 50 to a title, and use the Save List Order link, all the other position numbers are adjusted properly. If you have a list where the position numbers are NOT adjusted, you should report that bug here, with the URL of your list. (Moving a title Down, from position 1 to 50 for instance is a little trickier, but you should NEVER have to change EVERY title's position number.)

I'm not an IMDb employee, and have no insight about whether drag and drop will ever return. But there were a LOT of complaints when it was removed in December 2017, and you can find several ideas to up-vote when you search here for "drag and drop".
Lists: Drag and Drop needs to return
Drag-and-drop function not there when editing a list
Lists: Edit list enhancement requests
Editing Lists is still a nightmare
Why has imdb disabled the 'drag-and-reorder' feature in lists?

I am adding my 100 movies list and I put that new movie to 50th row and there was other movie in 50th row before that new movie.

Are you saying that you do the following, and get TWO titles with position number 50 afterward:

1. Add the title to the list.
2. In Edit mode, change the position number of the new title to 50.
3. Click the Save List Order link.

If so, please post the URL of your list and a screenshot showing the dual 50's.
This would be an important bug to get IMDb to fix.
It is not how the software normally works, so finding a list where it happens consistently would be important.

Example:
(I created a list with exactly 100 titles to simulate what you claim is happening.)
Here I've added the 101st title, and changed its position number to 50:

Here is what is at position 50 before I save the list order:



Here's position 50+ after I click Save List Order:


I changed only 1 title's position number. The software took care of the rest.

I wasn't know that I always did change all the rows lol. So that feature is crap we don't want that. I'm joking still we need that Drag & Drop feature. :)