The feature you're looking for is now called Home Sharing. It's essentially the same as the original sharing feature, but it ties into your Apple ID to ensure that you can only share between your own computers.
In iTunes, just go to File > Home Sharing > Turn On Home Sharing and enter your Apple ID and password. Repeat the same steps on any computers you want to share with (up to 5) using the same Apple ID. Once you do so, all enabled computers will "see" each other and let you stream or copy music between them.
regarding getting your SMB sharing connection from linux working...
Samba no more, mount.cifs needs extra options, "nounix,sec=ntlmssp"
Don't use the linux gui to connect, have bro open a terminal and try these commands
(and dig my ascii art!)
=^..^= `·.¸¸ ><((((º>.·´¯`·><((((º>
amitsbrother@linux:~$
amitsbrother@linux:~$ sudo apt-get install cifs-utils
...
amitsbrother@linux:~$ mkdir /mnt/mavericks_smb
amitsbrother@linux:~$ mount.cifs //172.17.8.212/smb_share /mnt/mavericks_smb/ -o user=amitsbrother,password=trustno1,nounix,sec=ntlmssp
amitsbrother@linux:~$
amitsbrother@linux:~$ mkdir /mnt/mavericks_smb_dup
amitsbrother@linux:~$ mount -t cifs //172.17.8.212/smb_share /mnt/mavericks_smb-dup -o username=amitsbrother,password=trustno1,nounix,sec=ntlmssp
Once this is working, you can create a script for your brother to automatically mount when it is executed from the gui. Basically, the script is just the mount point creation, and the cifs connection to the smb server... so 2 or 3 lines including the shebang.
Make sharing work now with no passwords
To make it super simple, I'd enable web sharing on the Mac, and put the files you want to share to the Linux box in a folder in ~/Sites/a_folder/ . Then give your brother the address that it tells is your personal web sharing address in the Sharing Preferences pane when you enabled Web Sharing. Tell your brother to open a browser and put in that address; it will give him a directory listing as long as there is no index.html file in there. He can download files with his browser. This is one way sharing, from the Mac to the linux box, and will work fine as long as there are no files over 4GB (unless apache fixed that issue and didn't tell me about it). I believe directory listing is enabled by default on the Mac apache2 server.
To share in the other direction, from linux to Mac, you could do the same from the Linux box:
sudo apt-get install apache2
You can enable directory listings on the Linux apache2 server with instructions here. Those instructions inadvertantly also cover how to get the apache2 server up and running. Then you need the ip address of the Linux box, and the relative location from the apache root to see the files in your Mac's browser.
This shouldn't take 5 minutes to set up 2 x 1-way sharing through browsers on both boxes, and relieves you from hacing to trouble-shoot the slightly more complex task of installing and configuring netatalk or running SMB sharing from the mac and getting the linux client to mount it, which isn't always a "it just works" situation, like running 2 apache2 servers is.
Best Answer
Regrettably I have found no solution to this anomaly. I was in the fortunate position to be able to erase and re-install Catalina.
The problem no longer exists for me. I have a “de-crufted “ system.
I’m always impressed by how OS X / MacOS has been able to upgrade this Mac successfully for about 6 years with very few of these kinds of anomalies encountered generally in upgrades over the last couple of decades and nearly a dozen different Macs.