I discovered the purpose of what looked like an NFC tag in my toothbrush heads. More expensive smart versions of my toothbrush log how many times the head is used, and start nagging about replacement after a while. Unlike with printer ink, it at least doesn't disable old toothbrush heads.
https://hackaday.com/2023/05/27/hacking-a-smart-electric-toothbrush-to-reset-its-usage-counter/
