At this summer's HOPE conference, Joshua Aaron spoke about ICEBlock, his iPhone app that allows users to anonymously report ICE sightings within a 5 mile radius, and to get notifications when others report ICE sightings near them. You can see the full talk, and the lively/infuriating Q&A, here,
It was BlueSky, not Mastodon and is linked in the article: https://bsky.app/profile/grapheneos.org/post/3lt2prfb2vk2r