It is not - it is in fact a nothing burger. The original claim was based on a German article where someone called Lufthansa and asked if they can put AirTags in their checked-in luggage, and the Lufthansa representative answered with the existing regulation about Lithium (metal or ion) battery devices, which says they are accepted if they are completely powered off.

That's it - no one is enforcing this regulation in practice, especially not for AirTags or other small devices.