To be more precise, this is one of the official search engines for the maintained classes only. The USGS no longer maintains a convenient search engine for all of GNIS, so we’ve hooked up iD to the tool that’s user-friendly and supports the majority of features. An ordinary mapper can still download the ZIP file of CSVs of archived features. (It isn’t very large by modern standards.) There are also third-party search tools, an official linked data distribution, and even a tile layer that you can load into iD, all of which include the archived classes.
The main use of GNIS feature IDs is to serve as a citation, a starting point for research. I have often used GNIS feature IDs of post offices (one of the archived classes) to restore a name that an overzealous mapper turned into a generic “United States Post Office” based on a validator suggestion. While I’m at it, the archived feature still has coordinates that are more up-to-date than what we’ve imported into OSM, and often a street address too. This would be very difficult to obtain without the gnis:feature_id tag on the post office node.
That said, if the relative inaccessibility of these archived classes matters to you, the best course of action would be to link the feature to a Wikidata item which in turn references GNIS (or create such an item if it doesn’t already exist). The GNIS feature ID automatically qualifies the item for inclusion under Wikidata’s notability guidelines.