Because of the decentralized nature of the IndieWeb, it’s most likely that more centralized services in the vein of Indie Map or perhaps a Microsub client might build in this sort of recommendation engine functionality. But this doesn’t mean that all is lost! Until more sophisticated tools exist, bootstrapping on smaller individually published sorts of recommendations like follow posts or things like my Following Page (fka blogroll) with OPML support are more likely to be of interest and immediately fill the gap. Several feed readers like Feedly and Inoreader also have recommendation engines built in as well.
Of course going the direction of old school blogs and following those who comment on your own site has historically been a quick way to build a network. I’m also reminded of Colin Walker’s directory which creates a blogroll of sorts by making a list of websites that have webmentioned his own. Webrings are also an interesting possibility for topic-related community building.
Since Tumblr is unlikely to shut down immediately, those effected could easily add their personal websites to their bios to help transition their followerships to feed readers or other methods for following and reading.
Of course the important thing in the near term is to spend a moment downloading and backing up one’s content just in case.