I actually don’t think that so-called abandonment should make an especially big difference here. If your “inspiration” is close enough to spur arguments about plagiarism, where you’ve changed so little that it’s recognizably an identical story, then that’s wrong even if the author hasn’t shown up on the forum for a decade.
If you’re inspired to write something “different enough” to avoid plagiarism accusations, on the other hand, then it doesn’t really matter if there’s another similar story actively in progress. Most people will probably accept it as something in the same subgenre and enjoy it even though it’s recognizably similar to another IF.
The key thing I’d like to note is that something left alone for
is probably but not necessarily abandoned, and definitely shouldn’t be treated as if plagiarism considerations don’t apply. About a decade ago, we had a forum conversation about this, where one forum member was arguing for “salvaging” the ideas from WIPs that had gone silent, comparing it to rescuing neglected children. I’ll just quote a few things I said then:
As writers, we can expect that people don’t plagiarize our work – but we can’t expect that people won’t be inspired to write similar pieces, whether we’ve just posted our work online or commercially published it. Once it’s out in the world, if people llke it, some of them will start trying to capture what they liked about it in their own writing. That’s part of how our stories take life.