Hello!
I am using the latest version of Insync (v3.8.7.50505) on Windows.
On this page https://help.insynchq.com/en/articles/3045421-ignore-rules , it says that “You can ignore both new and old files. Files that have been previously synced will be unsynced if it matches a rule.”
It does not seem to be the case if the file was synced before and the rule was added after the sync. A simple test can corroborate it.
Cheers!