breach of contract
on one level this is just a straight breach of contract.
employees make pension contributions over the years with a promise of RPI linked retirement benefits. the size of the contributions made are pre-calculated to be the correct level for delivering RPI linked benefits ( I worked as a trainee actuary for a while).
if CPI benefits had been promised, the pension contributions would have been smaller.
and when the employees comes to retire, the pension fund is now trying to break the original promise of the RPI link. this is what BT is trying to accomplish, although the pension trustees are fighting this. they have to fight this because pension trustees are personally liable
and RPI is definitely still a thing - examples are landfill tax and renewable energy certificates which both index at RPI for the next 20 years.