SOLUTION!!!
You need to be on iOS 13.4 first... earlier versions had a bug.
The issue is a corrupted Siri preference file not syncing correctly via iCloud to the Siri backend, leading to relentless polling.
The fix is simple.
1. Start with the iPhone.
2. Log out of iCloud on one device, log back into it, Merge when it asks you, then wait for stuff to repopulate (a couple of hours). (Do this one Apple iOS/tvOS/iPadOS device at a time, give the device time to repopulate.)
3. Then do your other devices one by one, slowly (as all your iOS/tvOS/iPadOS devices may have the issue).
No need to do MacOS devices from what we’ve seen here.
The battery drain should stop.
Also, after you plug in and charge once, the battery usage should display correctly and Siri won't appear at that ridiculously high level anymore.
UNLIKELY SECOND STEP:
If this *doesn't* fix things, go to General > Reset > Reset All Settings.
This will flush all pref files... you'll have to set things like Apple Pay cards back up again.
Then do the above iCloud logout/login... (Had to do this on an older (not supported anymore) iPhone 6 I keep about for testing.) So... unlikely that it will be required in never devices.