Currently if you create a cache entry on a system where gzip is used and then restore the cache on a system where zstd is used, you get a cache miss.
It would be nice if instead mutliple compression algorithms were supported.
Something like this:

on cache creation:

• zstd with --long present
  • use zstd with --long
  • end
• zstd present
  • use zstd
  • end
• use gzip

on cache restoring:

• zstd with --long present
  • if according cache entry is present
    • use that
    • end
  • if according cache entry is not present
    • continue
• zstd present
  • if according cache entry is present
    • use that
    • end
  • if according cache entry is not present
    • continue
• use gzip
  • if according cache entry is present
    • use that
    • end

This way, for example a cache entry created on a gzip system can be used even on a zstd system.

And to be even fancier, when restoring a "lower" level (gzip or zstd on zstd with --long, or gzip on zstd), the file could be recompressed with the "higher" level and a new cache entry for that higher level created, so that next time it can be used more efficiently.