That is unfortunate...
I had an unrealistic expectation, I thought they'd be able to support Manifest v2 for a long time, or find some kind of workaround.
They say Vivaldi is future proof because of their built in adblocker, but unfortunately it is not very good...
Pity. This is going to make Vivaldi's userbase implode. Ah well, hopefully Firefiox will put some work into customization.