Senior citizens with Traditional Medicare and a Plan D supplement will see increases in their Plan D costs in 2027. Both the deductible and the out-of-pocket cap will rise about 14% according to a formula written into law. However, premiums will rise unpredictably due to the Center for Medicare and Medicaid Services terminating a subsidy that had been part of Biden's Inflation Reduction Act.
For example, I learned today that the monthly premium for my Plan D will quadruple in 2027.

"On July 28, 2026, CMS announced it is ending a temporary program that has been quietly holding down premiums on standalone drug plans since 2025. That program is gone as of January 1, 2027 — and unlike the deductible, nobody yet knows exactly what it will cost you personally."

https://www.gentlemedicareguide.com/medicare-2027/part-d-changes