If it is about a decentralized index there's also YaCy.net [1] but I don't know how actively maintained the project is.

For me it made more of an enterprise-grade use case (e.g. for building a search for your own file servers or confluence) so I only tested it out a little. It's a huge java project, that's why I decided to go with searx back then...cause yacy was pretty hard to setup.

[1] https://github.com/yacy