Well, its definetely possible on activitypub. Every “app” built on the atprotocol takes data from a relay’s firehose and then indexes it and does all the algorithm stuff. There is a project (https://www.fediscovery.org/) that will let people build centralised apps with fediverse data. Although, I could just make an algorithm that looks for keywords a user may be interested in, in the posts database and show it to them, it just wouldn’t have every post to its disposal.