I restarted my server on 1.8 with the shopkeeper plugin, i made a shopkeeper i added items and the trade didn't work, But on 1.16.4 it worked fine. i don't know the reason why but i hope this gets fixed.
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.
