Storage Almost Full

"Storage Almost Full" is a warning provided by iOS and iPadOS when the built-in storage drive of an iPhone, iPod touch, or iPad is almost full. This may render such a device non-functional as they rely on the usage of virtual memory for memory management. The operating systems may delete streamed data or files redundantly stored on iCloud in an attempt to avoid this.

Issues
This issue tends to appear during large downloads, such as major software updates. This may require the user to backup the device contents and delete unneeded files. However, a memory-reporting bug has caused the "Storage Almost Full" warning to appear in some devices that have over half of their storage free, as recently as iOS 15.