How to disable com.apple.quarantine on Catalina
As a workaround, I'd create a Automator Folder Action, attached to the Downloads folder, with a Run AppleScript action, setting Shell: /bin/bash and  Pass input: as arguments, using the example bash code below,  until a more permanent answer comes about.

for f in "$@"
do
    [ ! -d "${f}.download" ] || continue
    xattr -d com.apple.quarantine "$f"
done






Note: The example bash code contains limited error handling. The onus is upon the user to add any error handling as may be appropriate, needed or wanted.