I found these two domain so far. Maybe there's more.I'm not entirely sure it's an indexing service, it looks more like a feed aggregator. If the domain gets spidered regularly it could certainly be used for indexing purposes, though having 1 domain link to all your backlinks is going to look a bit strange.
https://ktwh.lab.b2tech.us/
https://prs7.org/?page=1