They owe you nothing. They are posting code, not a promise to work for free for you forever.

Exactly, they're just posting code, but, simultaneously, if they put it in the public domain, they probably expect that the code will be used by others and from that moment, well, they are at least responsible for replying to PRs or close the ones they don't care about merging.

Once it's public and gains "customers" the responsibility grows a bit, otherwise don't post it at all publicly, after all, if you're not going to care about real world use cases, it's just as valuable as any toy project that hasn't seen the light of day.

This type of almost "passive gatekeeping" is the reason why we've formed several OSS projects we rely on. Yes, we incur maintenance burden but the predictibility gained is Invaluable.