I want to wish you that you put them in good use while having clear goals for the GRIN's future (so as to not waste resources).
It doesn't add anything to the discussion, and almost always congratulation posts are unneeded. I'm not saying you can't congratulate other users, but bring something new to the discussion rather than just saying congratulations, and its one of your favorite coins. Can you imagine how much of a ball ache it would be to read the thread if everyone did that after an update?
Even with your revised comment I believe it should be removed. It doesn't really bring anything with substantial value to the thread, and is just added on to avoid getting deleted. If a reply has been deleted, its probably not the best of ideas to continue posting it. There's also other ways of congratulating users of something, that doesn't end up clogging up the thread. You are not the only user who gets these types of posts deleted, and there are several users who like to congratulate project owners when they reach a certain milestone etc. When they are reported they are usually deleted, unless they are otherwise not low quality.
I consider most congratulations posts to fall under rule number 1 for being low quality. Like I said, you can congratulate a project/user, and at the same time be a substantial post. However, I don't think yours was. As for your posts on the DAPP thread, I have no idea without context.
As for the idea, its definitely something that would be beneficial. As others have stated it is part of newer versions of SMF. However, this forum runs on a highly modified SMF that doesn't have that feature. If it was a feature then I don't see any reason why moderators wouldn't take a couple of seconds more on each report to write a little something. Although, you could probably expect the obvious rule breaking posts that are deleted to not get an explanation.
Anyway, almost everyone on this forum has had a post deleted in their history including myself. It's usually not a big deal, and after a while you start to learn what sort of things you can, and can't post. Certain sections can be stricter than others, and certain moderators can be stricter than others.