I'd probably like the upvote itself to be considered "speech". The practical effect of upvoting is to endorse, together with the site's moderators and algorithm

I'd probably like the upvote itself to be considered "speech". The practical effect of upvoting is to endorse, together with the site's moderators and algorithm-curators, the comment to be shown to a wider audience. Along those lines then then an upvote i.e. endorsement would be protected, up to any point where it violated one of the free speech exceptions, e.g. incitement.