The point is about creating a mod, not using current exploits/bugs.
I want you guys to relate to the topic, i want to know if it is anyway possible.
I dont want to sound rude, i really just want to know if there is a way to do this.

The point is to have it on one belt and not use a smart inserter as it takes another 2 lanes.
There are some cases when you just need it. Btw the point of this post was not to talk whether it is useful for one other or not, but how to do it as of now.

The point is when you have a belt of coal and copper for example, and you want to take only copper,
but you have a single block of space to block copper, you need an item blocker to extract it.

Well, its too complex i guess, and could get laggy. Probably we should ask the devs to add a feature like this
or at least bring back the support to blocking the item path on a belt.
I'm new to modding, is it possible to create a belt with only 1 lane moving?

There was a mod once, Item Blocker(https://forums.factorio.com/forum/viewtopic.php?f=14&t=6227&start=0) , it contained an item blocker which blocked items on a belt from moving on the belt, while remaining static. I tried some tweaking with the mod, but got no luck in bringing its functional...