Knowing and voluntary appeals waiver in plea deal forfeited the right to challenge a sentence that, given subsequent developments in the law, has become in excess of the statutory maximum sentence for the crime. Circuit precedent to the contrary was dicta.
http://www.opn.ca6.uscourts.gov/opinions.pdf/18a0136p-06.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.