On a macOS computer running Catalina, is it possible to have the /Developer/ folder that used to be present since Mac Os X was born?
In macOS Catalina the new way is to use /etc/synthetic.conf to create a virtual symbolic link to the root of the e.g. Macintosh HD for the target folder. All writeable paths must reside on the data volume, which is mounted at: /System/Volumes/Data

It Terminal, run the following commands:
sudo mkdir -m 0777 /System/Volumes/Data/Developer
sudo nano /etc/synthetic.conf


Add the following entry to /etc/synthetic.conf:
Developer       System/Volumes/Data/Developer


Note there is a tab between the columns.


Save the file using: controlO enter

Close the file using: controlX

Reboot for the changes to take effect.


Note: While this creates a symbolic link of a plain folder, nonetheless, if you give the target folder  e.g. /System/Volumes/Data/Developer a custom icon and then restart Finder, (in Terminal killall Finder), it will show the custom icon on the symbolic link of the target folder.
 See: Create custom icons for files or folders on Mac
 For more information see the manual page for synthetic.conf, in Terminal: man synthetic.conf

Note: The Developer folder icon file is located at:
/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/DeveloperFolderIcon.icns

I added the custom icon by temporarily creating a Developer folder in my Home folder, which automatically gave it the Developer icon, and then used Get Info (⌘I) to copy the icon to the Get Info for the Developer folder at: /System/Volumes/Data/

Note: in Finder the Data folder will show as e.g. Macintosh HD when navigating to System &gt; Volumes &gt;.