Earlier quoted context omitted.
As far as I understand there is nothing new. Lithium batteries have always been allowed in checked luggage only if they are inside a completely powered off and reasonably protected device, to avoid the risk of fires. This risk doesn't really apply to such small batteries, but the rules are there regardless of the size of the battery.
AirTags don’t use lithium ion batteries.
Note that lithium itself is a very volatile metal (it starts burning in contact with water, for example).