There is currently no way to search within a specific plugin’s support threads only and due to the complexity of WordPress’s existing network, this is not likely to be addressed any time soon.
https://wordpress.org/support/topic/how-to-search-these-forums/
However, one can search among a particular plugin’s forum {topics} using a similar method to what @Milo posted above in the OP comments. We just need to modify the url a bit.
site:wordpress.org/support/plugin-name/ {query}
This will result in finding a {query} among the paginated support thread titles.
*Also note that since these results are all very similar url’s with just the page number being different, one needs to pay attention to Google omitting results and of course one must skim through each of these pages to find the topic matched.