Just search for any Norddeich-Bremen connection in the affected three months on the DB route planner with "only local transport" ticked and you will see the label on every second train. Well, after clicking "Show more information" and carefully reading the grey on white text.
Ok, that's apparently a special case, that's an Intercity train (as can be seen from the "Operator: DB Fernverkehr AG" on the page above) that can usually also be used with regional tickets, but DB apparently decided that doesn't apply to the 9 € ticket. The trains are painted in DB's "long distance" livery, but I'm not sure how many passengers would interpret that as a clue that the 9 € ticket may not be valid...
People are used to use a regional ticket with those trains, and they show an "RE" number. This will likely lead to loads of discussions on the train if they will follow through with that.
It seems that both IC train number and RE line number appear for these kinds of trains, so for savvy passengers it is possible to recognise such connections. Allowing local travel in selected IC trains is pretty interesting, though.
Edit: I tried in English and the warning didn't show up... Hopefully this link works as an example: https://reiseauskunft.bahn.de/bin/traininfo.exe/en/45678/209...