This uses Xapian. Some of the Linux mailing lists, like linux-netdev, use Xapian.

I use Xapian on the desktop but I only store text documents so I have no need for Recoll. It is refreshing to use search that is not tuned for "popularity" and selling online ad services. If search is too easy, then IMO it is not really search. Search should require some skill. It should not be a game where a commercial entity Hoovering up user data and selling ad services runs the search and tries to guess what the searcher is searching for.

I never tried it, but this looks somewhat similar to Recoll:

https://github.com/kendling/pinot-search