Do you have a link to the mastodon interaction where they threatened you with legal action?

I ask because I'd be pretty disappointed in GrapheneOS over that kind of thing and it'd probably at least partially change my opinion of them, but it's better to validate these types of serious accusations and get the full context.

I don't. My very vague recollection is that I was alarmed and either deleted it or blocked them. So it either no longer exists, but even if it does I have zero interest in digging it out. I'm always anonymous on social media like HN and Mastodon, but who knows what one can discover if they're the kind of unhinged person who will dedicate enough time to doxing someone...