Hey @[email protected] I'm sorry I missed this post along the way. You know, I think I'm going to go check on whether one can get notifications on posts from various communities or not, and if not, file an enhancement request. There's definitely some I'd like lemmy.ca to be a little more active about telling me about.
At any rate - regarding federation - the lemmy software at this point doesn't pull in old posts from a community. When someone on lemmy.ca subscribes to a community on lemmy.ml, then federation starts from that moment and that's all the timeline that lemmy.ca will know about. I think this is something that should get enhanced over time - I can understand not wanting to flood an instance with history on busy communities, but I can see that issue being remedied in other ways (either trickling old posts in over time or adding in federated search, etc)
So, with /c/embedded_prog - there's been no activity on lemmy.ml for 3 months, and I suspect the subscription here would have been after the last activity. So, if you post something to it, it should work and federate over to lemmy.ml. Any new activity on any of the old posts on lemmy.ml should federate those posts and comments over here.