It doesn't work for I2P due to its design, but for things like Nostr, it works well. Essentially, the goal is to build up a list of "known" reliable relays over time, while simultaneously blacklisting anyone who joins and proves to be unreliable relying on the statistic that collaborative individuals outnumber hostile ones in any sufficiently large cohort.

Of course, it's far from being 100% effective, but it mitigates the issue significantly.

Hostile entities generally have a lot of money they can use to perform a Sybil attack.