WebP is an extended container around the RIFF file format, and contains the RIFF header info. So any container that is built off RIFF, or supports RIFF, can at least interpret the container data that is RIFF compatible and will lose anything that has been extended upon.
If enough humidity over time gets in there, the salt can start caking and forming larger crystal clumps. However, the salt itself isn’t damaged by that process and will work fine if broken back up and used in the quality you need.
A best by date here would be a notice from the manufacturer that the product should be shelf stable at least that long before “degrading”.