The 10% rebate was definitely clearly stated on the website when I renewed in December, but indeed there is no reference to the rebate in the welcome letter. I kept a letter form two years ago and it is listed in that letter as one of three benefits: 15,000 points, 10% rebate and complementary weekend night. In the latest letter it's just the 15,000 points and the complimentary weekend night.
Let's wait and see... I am waiting for 42,000 rebate points from a stay that I finished 4 weeks ago. I will have to wait for another two weeks until I can reach out to the Ambassador helpdesk. I have renewed my membership using this option 4 years in a row and thus far I had to chase for the rebate in 80% of the cases :-(