GrapheneOS has plenty of evidence and they post it alongside their claims. They post it carefully though, and are willing to provide it to people upon request.
https://grapheneos.social/@GrapheneOS/116442796907613215
https://grapheneos.social/@GrapheneOS/116442754144530576
https://grapheneos.social/@GrapheneOS/116439834987996043
https://grapheneos.social/@GrapheneOS/116439798112845463
https://grapheneos.social/@GrapheneOS/116439747793648606
(the linked post in Mastodon is the one displayed with a bigger font, not necessarily the first at the top of the page.)