@itsfoss "Skiff developers claim it to be open source, and their source code is available on GitHub. However, as some readers noticed, the licensing might not be entirely open-source compatible."
Actually, Skiff claims only Mail is open source on their website: "Skiff Mail is open-source, uses open-source cryptography, and undergoes external security audits."
On Reddit, Skiff confirmed they don't open source their backend:
https://www.reddit.com/r/Skiff/comments/10yn8a5/are_all_parts_of_skiff_opensource/j811jki/