The corrupted chunks have been deleted and allowed to regenerate.  Unfortunately, the terrain generation engine has changed significantly enough since the chunks were first created that there’s still a discontinuity in those spots, but the over-all map health seems fine.

We have also upgraded to latest Spigot build, which hopefully has fixed whatever bug might have allowed a few chunks to become corrupted.  I can’t specifically pinpoint Spigot as the source, but the changelog had some entries that looked like this kind of bug might have been fixed.  I’ll try to get an old-world map generated and uploaded in the next couple of days.