@Gregor, OpenSource extensions are depending on the updates by their devs. Sometime it means a lot of work.
Apart of Stylus there are a lot of important Extensions Mv2 on the Google Blacklist in the store, but this don't mean that they will be eliminated when Google schutdown Mv2 next year, if the are updated to Mv3 until then, some will be (eg.Stylus), another not (eg. uBO), this depends on their devs. But you can still use Mv2 extensions in Vivaldi until the shutdown (if they remain in the store until then).
Anyway, in estensions not related to security and privacy, there are no differences for the user if it is Mv2 or Mv3.