I’m no expert with numbers, but I wrote this post a couple of years ago after doing a fair bit of research. Why it's so hard to get into an under 15% acceptance rate school
That post was based on numbers at Cornell. It seemed that about 15% of deferred applicants were accepted in the RD round. So I think that a deferral isn’t always a polite rejection, but I also don’t think it’s a high enough number for anyone to count on getting in after being deferred.
I also think there plenty of colleges where the deferred applicant really has almost no chance. I think U Mich is notorious for this. They seem to defer everyone these days, and I don’t think many of those students get in. I haven’t investigated that.
I am sure that a lot of tippy top colleges defer people they have no intention of admitting, as a means of soft rejection. There are so many children of alumni applying to tippy tops these days. They can’t admit all of them.