UPDATE: I ended up doing it like this, so this is no longer a high priority. (Though it would be nice to be able to disable Direct Share on a per-app basis.)
[NOTHINGYET] XPosed module to disable Direct Share targets on Android 10+
The idea is to implement the ReVanced patch below as an XPosed/LSPosed module.
The method for an app to publish Direct Share targets on A10+ has changed such that blocking via the intents firewall is no longer possible.
Direct Share targets can only be published via the Sharing Shortcuts API on A11+.