The common case of plugins seemingly going poof is lack of updates for >2 years, which makes them de-listed from search.
In this case however plugin is clearly missing. This is either technical glitch of some kind (not too likely) or it had been removed from repository. The usual reasons are security vulnerabilities and license violations, however there is no way to tell without first hand info from person who removed it.
The channel to inquire would probably be [email protected]