Some additional Info from Finnair on the changes:

"Our new Premium Economy travel class will be in operation during spring 2021. As the sales for the new travel compartment start approximately one year in advance, we need to re-accommodate some of our existing booking classes to be used with the new travel compartment and its fares.

By removing booking classes P, T, R and W now, we ensure that there are no overlapping bookings in wrong travel classes once the booking classes are reintroduced back to sales during spring 2020. This change also enables us to align our booking class structure with our partner airlines, who also have a Premium Economy travel compartment.

(...)

After this technical change, we will continue to offer similar price points as with the removed booking classes by continuously reviewing the fare levels of other booking classes.