It's a pity. He already had a really good NHL career and could have had a great one.
For the Oilers, his injury was kind of a disaster, as it forced Darnell Nurse and Adam Larsson into being the only experienced defencemen on the team (other than shut-down guy Kris Russell, who was often healthy-scratched), which then forced Holland to ante-up to Nurse's agent in 2021 and sign that albatross contract. The departure of Klefbom also likely led to the departure of Larsson (his best buddy), which further hurt the Oilers' D-core.
In fact, it's quite likely that Klefbom's injury is what cost the Oilers the 2024 Stanley Cup. But such is life.
Anyway, happy retirement, Oscar!