The custom blocks can't be denied.
I tried to deny some custom blocks, but they didn't work on the server(they can work in the local game) 

Environment
Windows
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Changed description:
I tried to deny some blocks, but 

I tried to deny some blocks, but 

Changed description:
I tried to deny some blocks, but they didn't work

I tried to deny some 

Added environment:
Windows
Changed summary:
Resolution: Unresolved → Invalid
Custom blocks still can't be used in 1.14.20.1
It doesn't support yet. You can check here. (BDS Tutorial)
It says
Use Add-Ons
You can use add-ons on your server that you've created or downloaded online.
You cannot use the following types of add-ons on your server:
Could a mod please close this issue.