It wasn't dropped as part of the refactor of the session handling, done during this cycle. But in practice IdeSessionAddin isn't referenced anywhere else, not even in the meson.build, so let's just drop it (in favor of IdeSessionItem).
I noticed this while looking for the code that automatically saves (saved?) the session during time when some page sent a
notify::<prop> signal, avoiding total loss of session when crashing. I haven't yet dug further though, so no idea if that code is gone too.