Do linked topics notify the author of the post replied to in the new linked topic? Apparently not in 2015 but things may have changed. No-one has replied below so I thought it would be ok to ask here.
I am not aware a message is sent, but there is a notation in the original topic such as
and we mods generally make a comment referencing the OP, and the OP gets a ‘note’ (via their avatar) they were referenced, by whom, and in what topic. The ‘note’ is flagged as
and clicking on it opens the detail.
If it was a new topic that got merged the old topic also gets locked automatically with a similar line added so the OP has information about what happened to it.
As may be obvious Choice uses Discourse with minimal tailoring to minimise admin overheads and relies on the Discourse project for features and fixes.