I would also say that probably this feature was nearly custom made for Reddit and there are few other sites that you would probably use it on. For this feature to be useful, a few things need to be true:

1. The site has a lot of content worth parsing such that it's worthwhile to limit your search to only that site.

2. The site itself has a garbage search functionality that you would not want to use instead

3. You need to be searching that site frequently enough that there is a need to shorthand it instead of typing out the long form syntax

4. The site must have content that is generally picked up best by a traditional search indexer e.g. mostly be text based in what you want to search

There are relatively few sites like that in existence. Probably if I mentioned the above requirements to anyone well versed in internet, Reddit would be the immediate first example out of anyone's mouth. Perhaps there are a few other sites that might meet the mark (Stackoverflow and Github come to mind in my example, or possibly other social media giants heavier in text based content as well. Or Hacker News, ofc). But most other large sites' search story is better enough that even then I would probably not naturally use this feature instead.