I'm currently going over my Lego collection, and it's both an exciting and daunting task. I would just like to voice my opinion here:
Notifications for changes in both owned sets and when a major change is done to parts owned (e.g. deleted or divided into a/b) is, in my eyes, a pretty important feature. I am manually adding all the sets and parts I own, and considering the time and work this takes, it would be pretty devastating if after a couple of weeks my entire part collection is inaccurate. It would feel like all this was for naught.
However, I would also like to say that mail notifications might not be the best way to do this (?). You guys have made a nice new notification center at the rebrickable site, why not just keep it there and like that it would not feel so much as "spam". I don't know about you, but when I Lego, I Lego. But when I don't, mails about Lego will be ignored until it's time to Lego again, and at that time I would just go to the Rebrickable site anyway and check the notifications there.
Thanks for reading my wall of text