Youtube Premium and similar services by other websites

Over the last few months, YouTube has slowly been rolling out videos with a higher bitrate that are locked behind a YouTube Premium subscription.
BiliBili has been offering something similar for a longer time called 大会员 (Big Member)

Are these considered paid rewards?

If the subscription is towards YouTube itself and not the artist, then no, it's not a paid reward. If the artist is offering a higher res version behind their YouTube membership, then yes, but in your case you seem to be talking about the former.