It looks like it's not possible to upvote community document replies.
For example:
https://community.esri.com/t5/arcgis-pro-documents/arcgis-pro-3-1-is-now-available/tac-p/1267721

There are cases where it would be preferable to upvote a reply, rather than post a new reply with "Thanks!". (Everyone who has subscribed to that post would get notified of the "Thanks" comment, which is a waste of people's time.)
Example:
https://community.esri.com/t5/arcgis-pro-documents/arcgis-pro-3-1-is-now-available/tac-p/1267721/hig...
Could the upvote button be added to community document post replies?