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Space station 13 is the best example.

It technically started closed source, but has been open source with countless forks existing in parallel for at least 90% of its lifetime.

AFAIK, SS13 had a number of source code leaks that resulted in popular forks as well. Not exactly the spirit of FOSS.

Not really. No servers based off of leaked source code have really lasted - they get nuked by the larger community.