I can't find any mention of paid versus free accounts in this study. It used to be the case that people who paid for Twitter were already able to hide their "likes", before Twitter just made "likes" hidden for everyone. I would be interested in knowing if the visibility change caused anyone to give up their subscriber status, i.e. those people who would pay extra because they really care about keeping their "likes" hidden.

Note that Twitter "likes" is still not private today in the sense that the original post authors can see who liked their post. I suspect people who were really sensitive to this visibility simply wouldn't engage with risky content to begin with.