Here is the story OP might be thinking of https://www.laquadrature.net/en/2023/10/06/the-beginning-of-the-8-december-trial-is-also-the-judgement-of-the-right-to-privacy-and-encryption/
So nobody was arrested for using signal, however it was considered as incriminating evidence during the trial that they protected their privacy and didnt want to give their im encryption keys to law enforcement
Here is the story OP might be thinking of https://www.laquadrature.net/en/2023/10/06/the-beginning-of-the-8-december-trial-is-also-the-judgement-of-the-right-to-privacy-and-encryption/
So nobody was arrested for using signal, however it was considered as incriminating evidence during the trial that they protected their privacy and didnt want to give their im encryption keys to law enforcement