How to stop one file from uploading to iCloud

If you add the filename extension ".nosync" to the name of a folder, this folder will no longer sync with iCloud.

You will see a crossed out cloud badge behind the name of the folder.

See the article Rename files, folders and disks on Mac - Apple Support

These are some items you should not rename:

• App folders and any items that came with your system, such as the Library folder. (If you change the name of an item and experience problems, change the name back. If this doesn’t help, you may need to reinstall the software.)
• Filename extensions — the full stop followed by a few letters or words that you see at the end of some filenames (for example, .jpg). If you change an extension, you may no longer be able to open the file with the app that was used to create it.
• Your home folder — the one with your name on it.

You can drag and drop that folder / file to other location see the article Work with folders and files in iCloud Drive - Apple Support

Read iCloud Drive FAQ - Apple Support

léonie wrote:

It may be an unsupported feature, but it is working well for me.

Then I will try not to become too reliant on it. But, since I've been working from home and storing huge numbers of work documents, often for only a short time, my iCloud usage has gone up a great deal.

It may be an unsupported feature, but it is working well for me. I think, I saw it first mentioned in a post by  FoxFifth, but I am not sure.

That's so cool! I never knew that.

You're welcome 😊