I don't know what happened if you switched versions or something but shopkeepers are fully broken. Shopkeepers when opened display no items due to a bugged version and the only time items show up is if you are on 1.16
This is not a server issue, this bug effects every single 1.8 server that uses the Shopkeepers plugin
Try to toggle your debug mode on and tell any errors or notes that it says
Ok let me explain why it isn't working
1. You said you switched your server from 1.16 to 1.8, this breaks a lot of things.
From prior knowledge of DragMC, I changed the version to 1.12, some things broke so I changed it back
Let me explain why the shopkeepers didn't work.
When your server is updated, some things in your shop has to be changed as items went through changes between 1.8 and 1.12, There is a corrupted item or errors in your Shopkeepers Shops that is returning Null thus causing the shop to not work. I know this because I reinstalled shopkeepers and pasted my shop and it didn't work. Try to look for a way to find the corrupted shop item and delete it to fix your server.
click the download button, its to the right of the edit button
You can't without boosting unless you upload the map as a schematic and paste it into the cubed server