Guys, I think I found out why songs from unscouted artists are STILL downloadable sometimes.
Basically, it's not a thing with Newgrounds, but the Geometry Dash game files.
The steps go as it follows:
1. Open the AppData folder.
1.5. If you did so by opening "Execute" and typing "%appdata%" and got sent to the Roaming folder, click on AppData at the top of the window.
2. Click on Local, then on GeometryDash.
3. Make sure you have the music you want to use (and make sure it was uploaded to Newgrounds), move it to the GeometryDash folder, rename it as the ID of the song you want to use. Any typos will lead to people downloading the wrong song later.
3.5. If the audio file is something other than .mp3, convert it to that.
4. Open Geometry Dash, go to the level editor, click the circle with the gear, then on Custom, then on Select Custom Song, enter the ID of the song you sent to the GeometryDash folder and click on Search.
4.5. Alternatively, click on Saved after clicking on Select Custom Song and search for the song you sent to the GeometryDash folder.
5. Click on Use, and it should be able to be used, even though normally that cannot be done.
Of course, downloading songs from unscouted users is disabled, but the guy who uploaded the song can follow these steps to use it. Also, people still can download songs from unscouted users upon selecting an online level, apparently.