Signal is based in the United States, enjoy having CIA and NSA reading all your messages.
Telegram isn't perfect, but it is infinitely better than Whatsapp because it doesn't belong to Facebook, and also isn't from the United States. Also it can be used by normies without problem, unlike Matrix or Xmpp or what have you.
Im not sure how up to date that roadmap is, but the pull request for group support in Mastodon has been open for 1.5 years now. And for reasons I dont understand it was made intentionally incompatible with Lemmy. So dont get your hopes up.
Thanks for the thorough investigation and for suggesting activitypub.academy. I found the problem and opened a pull request.
Let me know if you find any other incompatibilities with Mastodon (or other platforms). For example I only found out some weeks ago that Mastodon federates reports in a different format from Lemmy (which is supported from Lemmy 0.19.2).
Unfollow activities from Mastodon should be supported. Can you try to refollow and then again unfollow the affected communities? Also check the server logs at that time. Its also possible that other users from your instance are still following the communities.
Releases are always for backend and frontend together, unless mentioned otherwise. You can check the tags in the git repos, or check the available versions on docker hub.
That doesnt work because we generally cant trust remote servers. Plus we dont even know where to fetch from, so wed have to run a complete crawl of all known instances which isnt practical.
Images in Lemmy posts should be rendered correctly by Mastodon with this pr which federates the link content-type.
This is out of scope for Lemmy, there are already dozens of Fediverse projects which provide this functionality so it makes no sense to reimplement it in Lemmy as well. And if you want both follow both communities and users in the same platform, thats exactly what Kbin and Mbin provide.
That paper is eight years old and yet there has been no major hack of the Telegram protocol.