I'm having the same problem as the person who asked this question: I cleared a bunch of files out of my Trash and instead of getting free space on the volume, I got 30 gigs of "purgeable space". According to the accepted answer to that question, I can engage in an entirely reprehensible workaround to liberate this space from its ghostly, meaningless existence in purgatory—a strategy to which I may well be forced to resort.
None of the answers to that question, however, deal with the more fundamental issue of why this is happening in the first place. Ideally I'd like to avoid having to do this ever again. Is there some way to tell the filesystem not to mark files as "purgeable" rather than just removing them? I'm running High Sierra, and the system drive (the drive in question) is using APFS.
Best Answer
Based on this Apple support article, I gathered that the purgeable space might be taken up by local snapshots. I had already backed up since emptying the Trash, but I have more than one backup drive. I removed the backup disks that aren't accessible at the moment and some time later my system drive's purgeable space was shown to have been alchemically transmuted into free space.
Since Apple hasn't bothered to include information on how local snapshotting works when you have multiple backup drives, and since I never managed to discover a way to identify what is currently contained in purgeable space, I can't be entirely certain that this was what fixed the issue. It would be quite helpful if there were to be a post listing the various ways that purgeable space can come about and how to prevent this from happening. I haven't managed to find such a resource; in fact, the support article doesn't even include the word "purgeable"; perhaps comprehending such an arcane detail is considered to be beyond the mental capacity of Apple's target demographic, even for its support pages.