Defaults matter: Opt-in may be better than opt-out.
For opt-out, you only know that people who disabled it care enough about not wanting it to explicitly disable it. If it’s enabled, that could be either because they’re interested or because they don’t care at all.
For opt-in, you know that they explicitly expended a tiny bit of effort to manually enable it. And those who don’t care sit in the group with those who don’t want it. That means it’s much more likely that your feedback is actually appreciated and not wasted. Additionally, comments with extended voting enabled will stand out, making them more likely to catch feedback. (And there will probably still be plenty of comments with extended votes for passive learning.)
Defaults matter: Opt-in may be better than opt-out.
For opt-out, you only know that people who disabled it care enough about not wanting it to explicitly disable it. If it’s enabled, that could be either because they’re interested or because they don’t care at all.
For opt-in, you know that they explicitly expended a tiny bit of effort to manually enable it. And those who don’t care sit in the group with those who don’t want it. That means it’s much more likely that your feedback is actually appreciated and not wasted. Additionally, comments with extended voting enabled will stand out, making them more likely to catch feedback. (And there will probably still be plenty of comments with extended votes for passive learning.)