AirTags are allowed in checked bags.

https://www.faa.gov/sites/faa.gov/files/hazmat/packsafe/reso...

The battery would be under the "In Equipment" column, not the "Spare" column.

Nor do AirTags contain lithium ion batteries, they use standard non-rechargable CR2032 cells.

CR2032 batteries are lithium batteries, so they do fall under the rules for "Spares" (no to checked baggage). They are permitted "In Equipment" for checked baggage. They are on the "Lithium Metal" row as one of the three example photos.

For the curious: https://en.wikipedia.org/wiki/Lithium_ion_manganese_oxide_ba...

At some point, airlines were raising concerns, but the rules have changed.

When I recently pointed out to the lost luggage department at an airport that I knew for a fact that the luggage (or at least the tracker) was right at the airport, they did not appear to be surprised or concerned at all.