Amazon is changing its feedback removal policy. It made the announcement on March 23, 2018:
Upcoming Changes to the Seller Feedback Removal Policy
Mar 23, 2018
Currently, seller feedback is eligible for removal up to 365 days after the buyer submits it. Effective May 17, 2018, after 90 days seller feedback will no longer be eligible for removal. This policy change will apply to all seller feedback, including feedback submitted prior to this announcement. Feedback removal requests must still meet all other eligibility criteria to be considered for removal.
To learn more about managing your feedback, visit https://sellercentral.amazon.com/gp/help/761
The reaction from sellers seemed to be a collective shrug, with some expressing surprise Amazon had even had a policy where it would consider removing feedback for up to a year after it was left.