Same here! That was one of my favorite features.
Any updates on this?
Last I heard they (Samsung) were working on it, but no firm date. I would assume all resources are on their upcoming devices for now.
Figured it out! first step is to go into "My Files" find the audio MP3 you want as a notification. Click and hold it so that it gives you the option to copy the file. Then, search "Notifications" in the "My Files" Folder and an internal storage folder should be shown named "Notifications", open that file and paste the MP3 into that folder. you can then go into your sound settings and you should be able to select the file as an option. Best of Luck!
Thanks for sharing, Tom!