Thumbnails which are older than 6 months should automatically be deleted to force creation of new ones. Reasons:
- Updated image scalers: Many thumbnails were created with image scalers which are deprecated by now. There are plenty of examples where thumbnails older than 6 years are not sharpened (jpeg sharpening was introduced later, I think) or broken (Quite a few svg rendering issues were fixed over the years)
- Many bug reports about cache issues: There are many(!) bug reports open right now complaining about caching issues with thumnails.
- Manual purge does not scale.
- Clean up of unused data: I am not sure about that but possibly the deletion of old thumbnails makes free space available again on the hard drives.