So up until now Dropbox used zlib compression? I would have thought that they would use zstd or brotli for a good speed/ratio-ratio or lz4 for speed.
DivANS looks interesting/promising like a nice little kit for compression. Doesn't seem like it's good for archiving since it doesn't seem to have support for seek (or so it seems, would like to be proven wrong), but very interesting nonetheless.