Hello, thank you for reporting your issue with the output prefix.
What version(s) of the Spine Editor are you using when the output prefix does not function as intended?

It seems that you create a folder in Windows Explorer before exporting, but actually, you can create a folder at the same time as exporting by adding any folder name to Spine's Output folder. For example, let’s assume the original path was below:
/Users/<user name>/Library/Application Support/Spine/examples/4.1/spineboy/spineboy.png

You can add the name of the new folder you want Spine to create in the last part:
/Users/<user name>/Library/Application Support/Spine/examples/4.1/spineboy/ExportedPNG/spineboy.png

If you specify the path like the above and press the Export button, it will create a new folder and save the files in it if there is no matching folder. Since this is more efficient, I recommend specifying the path in this way unless there are special reasons. (Precisely, the more efficient and most recommended method is to export via command line interface: Command line interface - Spine User Guide)

Or do you have a particular reason why you need to create a folder in Explorer?

Sorry that my response confused you.

1. Are you suggesting that I memorize project paths?

No, you can specify the folder you want to create a new folder via the browse button, then you can enter the new folder name at the end of the path.

2. Why should I write spineboy.png?
I have 3-5-10 animations and they are not called by the same name. They are unloaded under their own names: [skeletonName-animationName.png]. What does it have to do with what I write in the path to the names in the final file?

You are right, in this case it was the specification of the Output prefix, so the <filename.png> was unnecessary. I got confused because the same field for PNG export changes to Output file when Current Pose is specified in the Export type field. Sorry about that.

CLI has nothing to do with the problem. I work in gui (Spine Editor), then for some reason I have to go to the command line?

The reason I mentioned the CLI is because it is actually recommended to save the export settings as a JSON file and export your skeleton via the CLI with the JSON file rather than exporting it via the GUI. For example, if a setting JSON file is saved after setting the correct path for the output folder each time, you can load it when the correct path is lost from the Output prefix, such as after editing another project.
Export - Spine User Guide: Saving and loading export settings

However, finding and loading the JSON file itself is somewhat of a hassle, so it is easier to write and save a command that can load and export the JSON file, especially if there is a possibility that it will need to be exported over and over again.

Sorry for being unclear on that point. I actually could not reproduce the problem on my Mac PC, but I have confirmed that the problem can be reproduced on Windows PC.
We are investigating the problem, so please wait a while for a response.

We found that there was a bug. If a skeleton had no skins, it would export as if "all skins" was selected. that looked like "multiple animations" needed to be exported by other code, so it wasn't doing the output prefix correctly. We will fix that in 4.1.

However, it was sort of intentional that the output prefix was not to select a folder, because we thought there might be a demand for customizing the prefixes of the output files. For example, compare the two different ways of writing Output prefixes below:
1.C:/skeleton/exportedPNG/
2.C:/skeleton/exportedPNG/anim
In the first case, the exported files will be named <animationName> + _<frameNumber>, e.g. walk_001.png.
In the second case, the exported files will be named "anim" + <frameNumber>, e.g. anim_001.png.
This is the reason why the field was named "Output prefix" instead of "Output folder".

The update has not been released, so I will inform you when it becomes available.

If you enter a path to a folder, Spine chooses the file names for you. If you enter a prefix, Spine only appends the frame numbers to your path. This allows people to choose their own file names, if they want to do so. It makes for some trouble with the file dialogs though, as we can only have the dialog choose a file (for the prefix) or a folder. We've changed it to choose a folder in 4.1.17, which will be released soon. The change also affects Spine on Linux.

FWIW, as Misaki mentioned, if you are doing many exports it can be useful to write a shell script to export them all with Spine's CLI. In exchange for the initial setup, this removes tedious clicks and ensures everything is exported correctly each time. It is also easy to export everything, to be sure the animations match the project files.