When does Multiuser mode no longer function in QB for Mac 2019

Hello, dypinc. 

Thank you for reaching out to the Community. Yes, the multiuser feature is still available in QuickBooks Desktop for Mac even though it is outdated. You'll need to ensure that you have the right file-sharing setup so the remote computer using your server can access the File. Also, check the multiuser setup has the company file stored and the hosting is turned on. To turn on Hosting, here's how:

1. Sign in to your company file as the admin on the host computer.
2. Go to File, then select Enable Multiuser Mode.
3. Click Yes on the confirmation window.

You can see additional details about this process and all the information you'll need to use your remote server in Use multiuser mode in QuickBooks Desktop for Mac. 

If you continue to have issues with your remote computer accessing the company file, you can check this article for more troubleshooting: Troubleshoot multiuser issues for QuickBooks Desktop for Mac. 

Additionally, I'd suggest visiting this website: Mac 2019 User Guide. Then, please go to pages 26-30. From there, you and your client will learn more about the QuickBooks Server and the other tasks to perform when using the multiuser mode. 

That should answer your concern for today. Post again if you have any other questions. I'll be here to help. Cheers!