Hello anyone potentially reading this. Given the very low demand for this mod, I have eventually decided that it is unfortunately no longer worth continuing to maintain this mod. Although disappointing for anyone potentially still using this mod, some of my "top" projects and new mod ideas also already take a lot of time and effort to maintain and work on, meaning it is next to impossible for me to direct my focus on mods that have "low demand". Below I will offer what I believe to be a helpful alternative to anyone needing help reducing their world sizes for latest game versions.

Aternos, also known as a free Minecraft server hosting service, has their own tool that they developed that serves the exact same purpose. The project is called "Thanos". However, keep in mind that this is only usable by those who are already experienced programmers and know how to install and use PHP and CLI. Click here to view the project.
For obvious reasons, not everyone is an experienced programmer who can just download "Thanos" and do what they gotta do. For this reason, I have made my own CLI script that uses Thanos, and that is "user-friendly". I have Windows and Linux versions of this script. All you have to do is run it, and the script will guide you through the process and do the world size reducing for you. Hopefully a useful alternative for anyone who might need it. Also, altho I made that script at the times of `Minecraft 1.20.1`, it should work fine on future versions. Up to what point I am unsure, but work on future versions up to some point it will. It may break one day, but, I have no idea when and if that will take place. Depends on Mojang I guess. Click here to view the script project.

PROJECT DESCRIPTION STARTS HERE:

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Regarding 1.19.3 and going forward: Many changes were made to the game with the release of 1.19.3, and Mojang has stated that they may bring new features in "patch" (1.x.x) releases now, and not just "minor" (1.x) releases. As such, please avoid using this mod on versions it does not specify it supports, as it could lead to crashes and (possible?) damage to world files (depends on the Minecraft update).

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This Minecraft mod reduces world size by discarding unused chunks.

Important: Please back-up your world if you are planning on using this mod on an already existing world. This mod shouldn't cause any issues, but it is still a good idea to back-up worlds, just in case.

How to use this mod:

Go to the world selection screen
Select a world of your choice
Click on the "Edit" button
Click on the "Optimize World" button
There should be a checkbox called "Remove uninhabited chunks". Check it it.
Click on "Create backup and Load". This will back-up your world just in case something goes wrong.
That's it. It's best to also verify everything went to plan and that no data loss took place.

A showcase of removing unused chunks

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This video shows how the mod can be used to remove "uninhabited" chunks to reduce world size:

[📦] No Unused Chunks